REPRODUCTION
I. Animal Reproduction.
§ 1. ASEXUAL REPRODUCTION.
AS a general account of this process has already been given (see Biology, vol. iii. p. 686), and the details of its occurrence in the various groups are described in the articles devoted to them (see Protozoa, Hydrozoa, Tapeworm, &c.), it suffices here to recall the very broadest aspect of the phenomena,—that asexual reproduction is simply discontinuous growth, and that, as we make an ascending survey of the Metazoa , that simple form of discontinuous growth which we term asexual reproduction becomes more and more subordinated to, and at last wholly replaced by, that more differentiated or “sexual” form of reproduction characterized by the union of two heterogeneous cell-elements—never to reappear save in degenerate forms. See Sex.
§ 2. SEXUAL REPRODUCTION.
In the lowest forms of animal life the process of sexual reproduction is found in its simplest imaginable expression unassociated with any of those complexities which arise among the higher animals and plants. All that is to be observed is the growth of the reproductive organs, the maturation of their products, aud the passive liberation of these,—the fecundation of the ovum and the fate of the embryo being entirely left to the circumstances of the environment. In higher forms, however, not only does this process of maturation become more complex and of far more marked influence over the other functions of the organism, but the attraction of the sexes becomes distinct and leads to new specializations of function and structure.
Maturation .—The maturation of the sexes acquires, as has been said, increasing definiteness in the higher forms, where it is associated with various characteristic accompaniments. The profound reaction of reproductive maturity upon the wdiole system is best marked in Birds and Mammals, and perhaps most of all in Man.
Thus in a young male Bird the circulation in the testes is greatly increased, and these organs increase greatly in size and weight and commence to develop spermatozoa. Meamvhile the “secondary sexual characters” of the adult—gayer plumage for alluring the female, or weapons for contest with other males—make their appearance, the voice and note may alter, and a marked increase of strength and courage may appear. Among Mammals the changes are of similar order, the secondary sexual char acters of course differing in detail. The minor changes at puberty in Man associated with the commencement of spermatogenesis are (besides the reflex excitation of erection due to distension of the seminal vesicles, and the more or less periodic expulsion of their contents during sleep) the growth of hair on the pubic region and later on the lower part of the face, and the rapid modification of the laryngeal cartilages and the lengthening of the vocal chords, so rendering the voice harsh and broken during the change and ultimately deepening it by about an octave. The marked strengthening of bones and muscles and the profound psychical changes which accompany the whole series of processes are also familiar. See Sex.
The local and cellular activity within the ovary, which is the fundamental part of maturation in the female, is not less remarkable than that in the testes. That even in lower Invertebrates the enlargement and escape of the ova are part of a normal cellular rhythm is interestingly shown by their not unfrequent relapse to the amoeboid state, or by the fatty degeneration and death of ova which have not accomplished their destiny. The escape of ripe ova in the Vertebrate ovary is especially remarkable: each Graafian follicle, as it ripens, bursts, expelling its ovum; its cavity contracts; it is filled up by blood, of which the white corpuscles form a ·framewmrk resembling connective tissue, in which the solids and corpuscles of the serum with colour״ ing matter derived from the haemoglobin of the latter are retained; and the whole constitutes the “corpus luteum,” which, should pregnancy occur, may persist and undergo further retrogressive changes, or otherwise gradually disappear.
The direct causes of this process of ovulation have been sometimes ascribed to the congestion of the blood-vessels of the ovary and to its own internal turgidity, or to the existence of a slight contractility of its stroma; it seems, however, rather to depend upon the growth and turgescence of the individual follicle. The question of the relation of ovulation to the process of copulation in the higher animals has also been much discussed. Though we certainly know that ovulation is of regular occurrence whether fecundation takes place or not, it seems that in many cases copulation is speedily followed by the liberation of an ovum; nor is it difficult to see how 7 the profound nervous and circulatory excitement associated with the former process might accelerate the bursting of a follicle. Leopold has conclusively showrn, how r ever, that ovulation may also long precede impregnation.
In many forms it is not easy to see Low tbe ova once liberated into the body-cavity find their way safely into the small opening of the discontinuous oviduct. In the Frog, however, tracts of the peritoneal epithelium become ciliated, so propelling the ova in the right direction. In Reptiles, Birds, and Mammals the open end of the oviduct is widened and fringed, and lies close to or even touching the ovary; muscular fibres too are present, and more or less active movements of this dilated end over the ovarian surface have been alleged to occur. The oviduct once reached, the downward progress of the ovum is ensured by the cilia of the epithelial lining, and probably also by peristaltic movements of its muscular coat. (I) Menstruation. —The process of menstruation (menses, catamenia), although from the earliest times the subject of medical inquiry, is by no means yet clearly understood. It occurs usually at intervals of a lunar month in all women during their period of potential fertility (fecundity), and, so far from being confined to the human species, has been observed at the period of “heat” in a large number of Mammals. Though thus clearly a normal physiological process, it yet evidently lies on the borders of pathological change, as is evidenced not only by the pain which so frequently accompanies it, and the local and constitutional disorders which so frequently arise in this connexion, but by the general systemic disturbance and local histological changes of which the discharge is merely the outward expression and result. The histological facts are briefly as follows. The mucous lining of the uterus consists of a loose vascular connective tissue covered by ciliated epithelium and containing numerous glands of clear alkaline secretion. This mucous lining before the outset of menstruation becomes loose and oedematous, its lymphatics being greatly distended; it thus thickens considerably, pressing against the cervix of the uterus. An extravasation of blood’ from the capillaries next takes place over the whole surface of the mucous layer, and the discharge is thus set up. This consists at first of blood largely diluted with the secretion of the uterine glands, but soon becomes mixed with detritus from the disintegration of the mucous coat, of which not only the general epithelial cells but those of the neck of the glands, and even part of the subjacent connective tissue, undergo fatty degeneration and fall off, occasionally even in a mass. After from three to six days the blood ceases to appear, and the lost epithelium is rapidly replaced, apparently by proliferation from the necks of the glands. By the ninth or tenth day the mucous coat is fully healed and the beginnings of the next menstrual process recommence.
The age at which the process commences varies with race and climate, with nutrition and growth, with habit of life (e.g., with differences between town and country life), and with mental and moral characteristics. Of these, however, climate seems most important; thus, while in northern Europe the average age is reckoned at the beginning of the fifteenth year, in the tropics it seems to commence in the ninth or tenth. The cessation of menstruation usually takes place between the age of forty-five and fifty, and, somewhat as the secondary characteristics of female puberty coincide with its appearance, a less distinct reduction of these is associated with its close; in many cases secondary resemblances to the masculine type may supervene.
The old theories of menstruation were that it served to rid the system of impure blood, that it simply corresponded to the period of “heat” observed in lower animals, or, later, that it was associated with ovulation,— which indeed seems broadly to correspond with the end of the menstrual period. At present there may be said to be tv r o rival theories. According to the first of these the process is viewed as a kind of surgical “freshening ”of the uterus for the reception of the ovum, whereby the latter during the healing process can be attached safely to the uterine wall. The other view is exactly the reverse of this. Its upholders regard the growth of the mucous coat before the commencement of the flow as a preparation for the reception of an ovum, if duly fertilized, and the menstrual process itself as the expression of the failure of these preparations, in short as a consequence of the non-occurrence of pregnancy. A decided majority of gynaecologists appear to incline to the latter view r . (1)
Comparative Anatomy of the Reproductive Organs. —The multicellular animals afford a remarkable series of gradations from the simplest imaginable case in which certain cells, independently of accessory organs, and even isolated from each other, develop into ova and spermatozoa. In the vast majority of cases, however, definite groups of cells are set apart as the essential reproductive glands—the ovary and the testis. The contents of these may simply break loose, but definite excretory ducts are very frequently present, and upon these very varied complications may arise. To the male ducts a seminal reservoir may be added. More or less specialized glandular regions may contribute their secretion to the seminal fluid, and a more or less complicated copulatory apparatus may also be superadded.
The female accessory organs are equally simple in principle and complex in detail. Nutritive material may be furnished to the ova by special yolk glands, or by the Avails of the oviduct; this too may supply special envelopes, and may exhibit dilatations for the preservation or development of the ova (uterus), for the reception of the male copulatory organ (vagina), or for the temporary storage of the seminal fluid thus introduced (receptaculum seminis). It is necessary therefore briefly to outline the most important facts of the comparative anatomy and physiology of these organs in the various groups.
Passing over the little-understood Orthonectida and Dicyemida (see Parasitism, vol. xviii. p. 259), the Sponges present the very simplest case above referred to. Here and there throughout the mesoderm a cell may be observed enlarging to form an ovum, or segmenting to form a mass of spermatozoa, but no definite reproductive glands, much less any duct or accessory organ, are present, and at most the ovum forms for itself a kind of nest among the surrounding cells, an approach towards the epithelial follicle of higher forms being thus presented.
Ccelentera. —In Anthozoa the generative organs are developed as ridges on the gastric septa, their products passing out by the mouth; and in the Ctenophora each radial canal bears an ovarian ridge on one side and a testicular on the other. Keen controversy has raged over the state of matters in the Hydromedusse. Kleinenberg derived both ovum and sperms in Hydra from the ectoderm, while Van Beneden endeavoured to prove the invariable origin of the ovarian tract from the endoderm and that of the testicular from the ectoderm; but subsequent observers, so far from confirming the constancy of this arrangement, have affirmed in many forms the ectodermic and in others the endodermic origin in both cases. Weismann and He Yarenne have recently very completely demonstrated the more remarkable fact that in a great number of forms the generative elements do not arise in the so-called generative buds or gonophores at all, but actually migrate thither from the parenchyma (coenen-chyma) of the nutritive polyp or trophosome—the latter observer indeed going so far as to allege the primitive origin by ova and spermatozoa in all cases from the endoderm of the nutritive polyp. These conclusions not only invalidate Van Beneden’s attractive theory, but tend to overthrow the ordinary view of the alternation of generations in the Hydromedusae. Vermes. —The incipient ovary and testis are seen in their simplest expression in such a case as that of the hermaphrodite Bryozoa, or even better in the low Cliseto-pod Worm Tomopteris (fig. 1), in which a patch of cells of the lining membrane of the coelom proliferate and enlarge into ova, or divide into spermatozoa, fall off, and become extruded. In higher Worms ( e.g., Lumbricus) the glands become localized in definite segments, and excretory ducts, apparently specialized segmental organs, appear. Among the Platyhelminthes , on the other hand, the most extraordinary specializations occur. The generative apparatus reaches a complexity which in some respects even excels that of the higher Vertebrates, and which, since it occurs not only among the parasitic forms but also in their less modified free-living allies (if not ancestors), the Turbellarians, must be regarded, not as any mere adaptation to parasitic life, but as an important factor in explaining (by the peculiar advantage which such increased reproductive efficiency affords; see Pakasitism) the wide prevalence of parasitism in the Platyhelminthes. Echinodermata. —Here the reproductive organs are of extreme simplicity—mere lobed glands usually provided with a duct or pore, or sometimes merely bursting into the body-cavity.
Arthropoda. —The essential organs never exceed one pair,—a simple median germ-gland being probably, as Gegenbaur suggests, the primitive condition. The accessory organs, however, reach great complexity, especially among the higher Insecta. Mollusca. —The lower Lamellibranchs have paired hermaphrodite glands opening in relation to the excretory organs; in the hermaphrodite Gasteropoda , however, high complexity of the accessory organs occurs {e.g., Helix). In the dioecious Cephalopoda the oviduct is single, and there are remarkable accessory female glands, while in the male the formation of the spermatopliores and the curious modification of an arm for copulatory purposes are noteworthy.
Tunicata. —In this group (so frequently hermaphrodite) the reproductive apparatus is again greatly reduced, the paired or single sexual glands being sometimes even ductless, while accessory organs are absent.
Vertebrata. —Starting again, in the lowest Vertebrates, with organs of an exceedingly simple and primitive kind, we find the series presenting all gradations up to a very high complexity. Thus in Amphioxus the reproductive glands are little modified patches of the lining epithelium of the coelom, and arise in a paired series,—an arrangement recalling that of the simpler segmented Worms. In other Vertebrates, however, the essential organs are most distinctly localized in origin, never exceeding a single pair, and are also much more evolved in structure. In its earliest recognizable state the generative gland is a slight thickening of the peritoneal epithelium and the subjacent connective tissue on each side of the mesentery, lying near and parallel to the incipient renal apparatus.
Leaving the histological details of the process by which this “germinal epithelium,” with the subjacent connective tissue of the “genital ridges,” develops into ovary and testis with their characteristic products to the sections dealing with oogenesis and spermatogenesis respectively, and confining ourselves to the gross anatomy of these essential organs, we may note their very simple character in the Marsipobranchii, where in the Lampreys they extend for a great length along the coelom, and exhibit a number of tolerably regular lamellar folds, recalling the segmental arrangement of lower forms. In the remaining Vertebrates these organs are usually less elongated and relatively smaller and more compact, though it is interesting to note that some Amphibians (notably many Cseciliae) exhibit traces of a more or less discontinuous, perhaps serial, arrangement.
In Plagiostome Fishes the sexual glands arise along only a portion of the genital ridge, the remainder having its stroma mainly enlarged, and forming the so-called epigonal gland. In many of the higher Amphibians accessory organs of unknown function, the so-called fatty bodies or corpora adiposa, are attached to the ovaries and testes; and in the Toad and other Anura another organ in close relation to the testis, and in histological structure resembling a rudimentary ovary, has also been described.
A considerable tendency towards loss of symmetry appears in the essential organs, particularly the ovaries, of many Vertebrates. Thus in Myxine neither ovary nor testis is present on the left side, and in many Sharks and Dogfish the left ovary is rudimentary. In Snakes the left ovary is smaller than the right and usually lies behind it, while in Birds the left ovary is alone functional, that of the right side becoming so completely atrophied at an early stage of development that traces of it in the adult are only found surviving in a few forms, especially some of the diurnal birds of prey. Among Mammals symmetrical development is the rule, yet in the curiously Bird-like Ornithorhynchus the left ovary is much smaller than the right.
The relative size of the ovaries varies greatly throughout the Vertebrate series, in relation partly to the relative proportion of stroma to germinal tissue in the histological structure of the organ, to the fecundity of the species, and to the number of ova produced, partly also to the presence or absence of a food yolk, and the consequent size of the ova. In many forms a great increase of size takes place at breeding-time.
In the majority of Vertebrates, as in lower forms, the essential organs remain throughout life in the position in which they develop, or at most depend into the coelom supported by a mesenteric fold. In most Mammals, however, a certain change of position takes place, the ovaries usually shifting more or less backwards towards the pelvis. The testes too in the Monotremata leave their embryonic position at the inner edge of the primitive kidneys and travel backwards. In Edentates, Hyrax, Elephants, and Cetaceans they remain near or a little below the kidneys, but they usually reach the abdominal wall, which they may more or less completely pass through in the inguinal region (as in many of the lower Rodents and Carnivores), or even descend into a more or less distinct diverticulum or hernial protrusion of the integuments of .the abdominal wall, the scrotum. This protrusion arises usually at the posterior margin of the primitive urinogenital opening, but, by exception, in Marsupials in front of it. The cavity of the scrotal pouch may remain throughout life in continuity with that of the abdomen, so enabling the testes to pass backwards and forwards at each breeding season (Marsupials, Rodents, Insectívora, Bats, &c.), while in the higher forms, e.g., Ungulates, Primates, &c., the scrotum retains the testes permanently shut off from the abdominal cavity.
The origin and homologies of the genital ducts in i Vertebrates, and the accessory organs in relation to them, may now, after a very great amount of anatomical and embryological inquiry, be considered as tolerably settled, ׳ at least in their main outlines.
Id J fyx'ine and the Lampreys no ducts are present, but the generative organs void their contents into the coelom, whence they pass out by the abdominal pore; some Teleosteans too (Salmonides) and at least one Elasmobranch (Lxmaryus) exhibit the same primeval simplicity of structure and function. Even here the exit of the sexual products is hardly independent of the aid of the excretory system, since there is reason to regard the abdominal pores as the least modified survivals of segmental organs; and in all higher forms the definite efferent ducts which are present in more or less close relation to the essential generative organs develop in the closest relation to, and in fact at the expense of, the renal excretory apparatus. For an account of the complex details of this process in the higher Vertebrates, however, the reader must consult the classical monographs of Balfour and Semper, or the larger manuals. (2)
Copulation .—We have noted above the importance of the copulatory process to secure fertilization of the ovum, and can thus readily understand its occurrence in the higher members of all the more complex animal groups. Though the result is in all cases the same, the process presents curious variations in principle as well as detail. Thus ( e . </.) the hermaphrodite Earthworms become firmly attached by their characteristic thickened band of fused rings ( clitellum ). Among the higher Crustaceans the spermatozoa are conducted to the ova along the grooves of a modified pair of the appendages of the male, while in Insects the modifications of the posterior abdominal segments and their limbs for copulatory purposes are often extraordinarily complex and varied. In Spiders, again, the spermatic fluid is passed into a receptacle in the chela, and thence pushed into the cloaca. In the higher Mollusca, the complex copulatory apparatus of the Common Snail and the process of hectocotylization among the Cephalo-pods, so curiously analogous to the process in Spiders, are too familiar to need more than mention (see Mollusca, Cuttle-fish). In many Fishes no copulatory process exists; thus in any of our Salmon rivers the male fish can be seen voiding the milt upon the ova after their deposition. In many Elasmobranchs a portion of the posterior pair of limbs, presenting very peculiar cartilaginous and glandular structures, though known as “claspers,” seems to be introduced into the cloaca during fertilization. But it is among Amphibians that we find the earliest trace of a true penis; a portion of the cloaca is distinctly ever-sible in Caecilians; in Snakes and Lizards paired eversible processes arise from the posterior cloacal wall, while in Chelonians, Crocodiles, and most Birds it is the anterior wall which bears these processes. In Monotremes, too, the organ is distinctly double; in higher Mammals it is single; but the function is in all cases essentially the same. The nervous, muscular, and circulatory mechanisms of the process are described in works on human physiology. (3)
Gestation and Birth .—While in the majority of lower forms the offspring leaves the parent as an unfertilized ovum, we have seen even among Sponges the impregnation and development of the embryo in its primitive position, and thus almost from the outset of an ascending zoological survey we can recognize the passage from oviparous to viviparous forms. The Invertebrates, however, are mainly oviparous, despite a few exceptions, of which perhaps the most surprising and aberrant are that of Entoconcha mirabilis, which exhibits an ordinary Molluscan development within the body of its Holothurian host, and that in what resembles a special ovarian tube, but is really the body of its utterly degenerate parasitic parent. Among
Insects a certain degree of viviparous development may be reached; and this goes curiously far in the Dipterous Insect Gecidomyia , in which larvae develop within the body of their parents (themselves at the larval stage), the cavity of which they destroy and burst in order to become free. Thus within the same species there comes about exactly the state of things in which the ova of a parasite develop at the expense of its host.
Among Fishes viviparous birth occurs more commonly: in some Teleosteans the young develop within the ovaries; in many Sharks and Dogfish the development takes place within the oviduct, and in one case (Mustelus Isevis ) an actual placenta is formed by the interdigitation of folds of the yolk sac with those of the oviduct. Even the terrestrial Amphibians usually lay their eggs in water, yet in some types, notably the Alpine Salamander (Salamandra atra ), development takes place within the oviduct. That this is a clear case of adaptation to the eminently terrestrial environment has indeed been well shewn by experiments in which the young larvae taken from the parent and transferred to pond water developed like ordinary Newts. To all such forms, viviparous in the sense of bringing forth their young alive, the somewhat confusing term “ovovivi-parous” is often applied. Birds, and also Reptiles, with few exceptions, of which the Ichthyosauria seem to have presented a striking case, are oviparous; so too, as has been recently established by Caldwell, is the in all respects so curiously Bird-like Mammal Echidna. Its congener Orni-thorhynchus probably agrees in this; but in Marsupials the embryo is not born until it has reached a comparatively advanced state of development, when it is transferred to the brood pouch or marsupium, where the process is completed. In the remaining Mammalia intra-uterine development goes much farther, the nutrition of the embryo being, in absence of the abundant food yolk of lower forms, effected by the aid of a placenta analogous but not homologous to that of Mustelus , since developed, not from the yolk-sac, but from the allantois (see Anatomy and Mammalia).
The physiological processes of birth show a similar rise in complexity,—due chiefly to the increasing strain upon the parental organization which this progress in the nutrition and protection of the embryo during its development involves; for, while an ovum can be extruded by simple ciliary action, or at most by the gentle contractions of the oviduct, the expulsion of the relatively enormous Mammalian foetus involves mechanical difficulties of the most serious kind. And, besides these stresses and strains upon the pelvic basin itself or the muscular and connective tissues of the uterus, vagina, and its outlet, the inevitable rending asunder of the large closely interwoven and highly vascular placenta must evidently occasion an additional physiological disturbance. (4)
Parental Care .—Not to mention cases of mere concealment of the ova or construction of egg cases, the lowest forms exhibiting such parental care are probably certain Holothurians and Starfishes described by Sir Wyville Thomson during the voyage of the “Challenger,” in which the developing young are borne upon the dorsal surface of the parent. Many Crustaceans carry about their ova during development, and an Amphipod has been described as followed by its newly-hatched young like a hen by its chickens. The female Spider too, though ferocious towards the male, frequently spins a nest and shows some maternal solicitude; but such cases are far commoner among even the lower Vertebrates than the highest Invertebrates. Thus among Fishes the case of the nest-building Stickleback is especially familiar; some Siluroids and Lophobranchs (and usually the males) carry about their young,—the latter in ventral pouches, the former in the mouth.
But the quaintest examples of care of offspring are those presented by some of׳ the Amphibians, notably by the Frog Alytes obstetricans, where the male winds the string of ova as laid round his body, sits in concealment until their development is sufficiently advanced, and then takes to the water, or in the grotesque Surinam Toad ( Pipa suri-namensis), where the male places the ova one by one in hollows in the loose skin of the back of the female, where they accomplish their development. Reptiles rarely show any care beyond at most burying their ova, but in Birds the evolution of parental care (no doubt associated with the need of high temperature for development) approaches its highest and most general evolution. The case of Mammalia is also too familiar to need any description; but there is evidently good ground for the idea (of late ably popularized by Miss Buckley) that the marked success of Birds and Mammals in the struggle for existence is to be attributed perhaps not less to their peculiar care of offspring than to any advance in organization. (5)
The Spermatozoon. History. —In 1677 Leeuwenhoek’s attention was drawn by Hamm, one of his pupils, to the active moving constituents of the seminal fluid, and he described these as animalcula spermatica or spermatozoa ( (nrepya , seed; ¿¿¡W, animal). Although known to be of constant occurrence, they were long regarded as parasites, and classified as Helmintlies or as Infusoria (see also article “Zoophytes,” Encyclopaedia Britannica , 8th ed.), even Yon Baer maintaining this view as late as 1835. Soon after this date R. Wagner demonstrated their constant presence in fertile males and their absence in infertile bastards. Von Siebold discovered them in many Vertebrates, while Kolliker recognized them as definite histological elements arising within the testes. The old name, however, has persisted despite various proposals to replace it ( e.g ., spermatozoids, Yon Siebold; fila spermatica, Kolliker). (.6)
Anatomy and Physiology of Spermatozoa. —Without attempting completeness it is necessary to note a few of the best marked forms of spermatozoa. The familiar type, that of a small more or less ovoid “head,” with delicate thread-like cilium or “tail,” is of the commonest occurrence throughout the animal kingdom; it is seen with specific modifications in 1-4 and 12-16 in fig. 3. Much less differentiated forms, however, occur, especially among the Branchiate Arthropods, of which some exhibit almost amoeboid forms,as in the Daphnoid Moina, or in Crabs (7 in fig. 3); others are rigid, with radiating processes, as in the Lobster (8 in fig. 3). That of Ascaris is a small nucleated cell without tail or process, but bearing a cap of protoplasm; this, like the lateral process in the spermatozoon of the Chaetopod Cletci (4 in fig. 3), is not, as might at first appear, an additional or accessory portion, but a mere persistent embryonic structure, as will be understood from the outline of the facts of development given below. In a large number the nucleus is more or less drawn out to form either a continuous rod or a series of fine granules, the protoplasm in such cases forming a delicate film with lateral fringes, which may also be produced on one or both ends into a filament. This form is seen in many Planarians (compare 9-11 in fig. 3). In the Newt the head is elongated, and the tail bears a vibratile membrane. In Man a middle piece separates the peculiarly shaped head from the tail, and this seems to end in a still finer filament (14 in fig. 3). Miescher and others describe their minute structure with yet greater detail, distinguishing in the head a wall enclosing peculiarly differentiated contents, in the middle piece a similar structure, and even an axial filament in the tail.
The movements of spermatozoa have attracted considerable attention, their action being mechanically comparable to that of cilia, and being affected similarly by reagents. Their great activity and prolonged vitality are noteworthy; thus not only do they remain in movement for many hours—indeed until the onset of putrefaction—after their ejaculation from the body, but in many of the higher animals they may remain active in the oviducal passages for weeks. In the Bat the spermatozoa lie in the uterus awaiting the ovum from autumn till spring; while a queen Bee may go on laying the fertilized eggs (from which workers arise) for swarm after swarm for three years after her first and only fertilization, without entirely exhausting the supply of active and mobile spermatozoa in the, receptaculum, and such cases forcibly suggest that the spermatozoon has not only a vast intrinsic store of energy, but must also absorb nutritive matter from the environing tissues or secretions, much in fact as would an Infusorian parasite. (6)
The chemical analysis of spermatozoa, though as yet but rude, yields results of interest. Thus, not only is the proportion of solid matter to water extremely high, but the small quantity of ordinary albumens and the high percentage of ethereal extractives are to be noted, the whole composition being more analogous to that of brain and nerve than of any other tissues. In round numbers, nearly 50 per cent, of the Salmon milt is nuclein, more than 25 per cent, is protamin, 10 per cent, is a mixture of ordinary albumens, and less than 5 per cent, is fat; while 1\ per cent, of lecithin, per cent, of cholesterin, with traces of other products of metabolism, make up the rest. (7)
Development of Spermatozoa ( Spermatogenesis ).—Probably no subject within the whole range of histology (at least if we except that of the structure of striped muscular tissue) presents so many difficulties, or has been the subject of more prolonged research and controversy; and it is thus essential to clearness to recapitulate the history of the inquiry in some detail. The modern period of investigation, despite a few observations by V agner and others, practically opens with Kdlliker’s fundamental observation (1841) that the head of each spermatozoon arose from the metamorphosis of a seminal cell.
In 1844 Haeckel described in Helix the cells destined to become spermatozoa as arising on the surface of an epithelial cell of the germ follicle. In a more elaborate paper (Z. JV. Z., 1856) Kolliker compared the process of spermatogenesis in the Bull, Dog, and Babbit with considerable detail, distinguishing in all cases the lining of the tubule into two kinds of cells,—the outer having large nuclei and nucleoli, and undergoing rapid multiplication, while the thick inner layer of smaller cells was becoming differentiated into the true sperm-cells, which might be either unicellular or multicellular “cysts.” He also described the origin of the head of the spermatozoon (and indeed also that of its tail) from the nucleus of this parent cell, and the breaking loose of the spermatozoon from its parent.
Henle (1865) showed that the tail was developed from a portion of the protoplasm, thus preparing for the comparison with ciliated epithelial ,cells soon afterwards instituted by Schweigger-Seidel. Sertoli (1865) described certain “ramified cells” in the seminal tubules, and Merkel (1871) regarded these as forming a framework (Stiitzzellen) for other cells from which the spermatozoa developed. In the same year, however, appeared the important researches of Yon Ebner, whose views will be understood by the aid of fig. 4. He described the large cells just mentioned as provided with a large and well-defined nucleus and nucleolus, and as being confluent at their base with each other, so forming a protoplasmic layer ( Keimnetz ), but stretching forwards into the lumen of the duct as irregular prolongations; these become lobed or fingered, each lobe independently developing a nucleus, the large primitive nucleus remaining unchanged. To the whole prolongation with its lobes he applies the term spermatoblast. From each lobe a spermatozoon develops, the nucleus lengthening to form the main portion of the head, while a thin film of the protoplasm elongates into the cilium or tail. The young spermatozoa at first press downwards into the body of the spermatoblast, so as to form bundles, but, when ultimately liberated, curl themselves up to roll down the seminal tubule, the more or less branched stump of the spermatoblast alone remaining along with the smaller cells of the tubules, which, according to him, take no part in the process, but merely perform nutritive or mechanical functions.
A vigorous controversy at once arose, but Merkel claimed these spermatoblasts as being simply his “Stiitzzellen,” and described the spermatozoa as arising from the small round cells, and as being only secondarily received into cavities of the former, a view which Henle also adopted. Sertoli also regarded Ebner’s spermatoblasts as his “ramified cells,” and ascribed all spermatogenetic functions to the round cells, which he distinguished in their youngest state as “germinative,” and later as “seminiferous” cells, which then divided to form “nematoblasts,” these directly developing into spermatozoa. Blumberg (1873) attempted to reconcile the disputants by ascribing spermatogenetic functions both to the spermatoblasts and to the round cells. Neumann, while supporting Von Ebner in the main, describes his spermatoblasts as not processes of the nutritive network (“Keimnetz”), but as modified from the ordinary epithelium of the seminal tubule, and disputed the existence of Merkel’s framework altogether; and this essential confirmation of Yon Ebner’s view was supported by Mihalkovicz, Landois, and others. In a later paper he attempted to show that the lobes of Von Ebner’s spermatoblasts might become separated off as the small round cells, and might then cither break up or develop into spermatozoa. Krause (1876), while otherwise supporting Von Ebner, graphically described the spermatoblasts as ciliated cells with ramified and even anastomosing processes, so doing away with any connective-tissue system altogether. Sertoli, however, continued to support his own view with greater elaborateness than ever.
Semper’s well-known researches (1875) on the urinogenital system of Elasmobranclis included an important contribution on spermatogenesis. He described an invagination of the germinal epithelium or of the primitive ova into the subjacent stroma, where they form a primitive follicle, which again comes into relation with the incipient tubule. The central cell of the follicle undergoes mucous degeneration and becomes absorbed, leaving a cavity lined by a single layer of epithelial cells, which divide rapidly, becoming cylindrical, with large round nuclei. These divide into a “ covercell” and a “mother-cell”; the nucleus of the latter divides repeatedly until about sixty sperm nuclei are formed, w r hich elongate to form the heads of the spermatozoa. As these develop they come to lie in a bunch, lying into the cavity of the follicle, their expulsion being effected by the swelling of the nucleus of the cover-cell.
An important series of researches by Von La Valette St George on spermatogenesis among Vertebrates was meanwhile in progress. In his final paper (1878) he describes the seminal tubules as containing tw T o distinct types of cell. One kind, resembling young ovules, which he terms primitive seminal cells or spermatogonia, divide into a mass or spermalogemma of small cells (spermatocytes). These spermatocytes may either (a) all develop into spermatozoa (Mammals), or ( b ) a single spermatocyte may become modified as a basilar cell (Plagiostome Fishes), or (c) a number may form an envelope or cyst around the others (Amphibians and Fishes). The second kind of primitive cells he terms “follicle-cells,” and regards as non-essential; these are united into a loose tissue and envelop the spermatogonia and spermatogemma.
In this view we have a marked divergence from both preceding theories, as the author does not hesitate to point out. He regards Von Ebner’s “Keimnetz” and “spermatoblasts” as confounding both his follicle cells with their included sperrnatogernmre, and interprets Neumann’s figures confirmatory of Von Ebner in the same way. Merkel’s “Stiitzzellen” he identifies as his “follicle-cells” altered by reagents.
The subsequent researches of Helman (1880), Krause, and Nussbaum among Vertebrates need only be mentioned as essentially confirmatory of the observations of La Valette. Meyer’s careful observations (1880) also led him to the same general view', with the important difference, suggested by one of the preceding authors, that the “follicle cells” of La Valette were not inert as he supposed, but the earliest stages of his spermatogonia and spermatogemma;. Klein’s views (1881) have perhaps most relation to those of Sertoli.
Some important work was meanw'hile being done among Invertebrates. In 1877 F. E. Scliultze described spermatogenesis in the Sponge Halisarca, —a germinal cell, analogous to that which gives rise to an ovum, becoming covered by an epithelium, and dividing into a multitude of segments, each of which becomes drawn out iuto a spermatozoon. In 1844 Meckel had described the spermatozoa of the Snail as arising superficially from a mother-cell, and this view had been confirmed by various authors. In 1879 Duval described in the same animal the spermatocytes as arising by the endogenous division of a mother-cell, and subsequently coming to its surface, and Hallez described an essentially similar process in some Planarians. Graff, how'ever, found in other species that no remnant survived, but that the whole of the spermatogonia became converted into spermatocytes. In 1880 Blomfield investigated the process in the Earthw׳orm, his results resembling rather those of Von Ebner (fig. 4) than those of La Valette. Strongly emphasizing, however, the importance of the nutritive basal cell or spermatoblast of Von Ebner, he proposed yet another nomenclature. In a subsequent paper (1882) he describes the process in the Snail and the Frog. The former agrees substantially with the Worm, but in the Frog a hollow spermatogemma arises; each of its cells elongates to form a spermatozoon; these while still immature arrange themselves in bundles round one of the more superficial cells, which “become blastophoral corpuscles,”—a view' which recalls Merkel’s explanation of Von Ebner’s spermatoblasts, mentioned above, viz., that the spermatozoa only became temporarily lodged in their recesses after completing their development.
In this respect the view of Renson (1882) is especially interesting. He describes the follicular cells of La Valette as segmenting into nematoblasts group themselves round the extremity of certain long projecting epithelial cells, the supporting cells (or “cellules de soutien,” obviously the spermatoblasts of Yon Ebner), and actually sink into their protoplasm to complete their development. When fully developed, the heads of the young spermatozoa have attained the base of the supporting cell, but this now elongates and bears them anew to the lumen of the duct, where new nematoblasts are by this time waiting to take their place (fig. 5).
The researches of Hermann (1882) in Elasmobranchs are broadly confirmatory of those of Semper, while those of Jensen and of Swaen and Masquelin (1883) are especially corroborative of the views of La Valette. The latter are, however, of importance as tending towards reconciliation. The primitive ampullae being formed, the “male ovules” and the follicular cells are henceforth distinct: the former segment into spermatogemmae; the latter (at first forming incomplete envelopes to the male ovule) mostly disappear, save one which travels downwards until it lies between the wall of the ampulla and the spermatogemma, thus constituting its “basilar cell.” The spermatogemma meanwhile is developing a central cavity (“loge caudale”), from which the tails of the incipient spermatozoa or “nematoblasts” project into the lumen of the duct. The basilar cell has also been enlarging, and fusing with the intercellular substance of the spermatogemma; the nematoblasts thus come to be plunged into the basilar cell, and sink downwards towards its nucleus; but this again elongates to expel them. In the Salamander these follicular cells form a complete envelope to the male ovules during their whole segmentation and subsequent evolution. In Mammals the male ovule divides into an active and a temporarily inert portion (follicular cell of La Yalette, germinative cell of Sertoli and Renson); the former segments into the spermatogemma, of which the resultant nematoblasts plunge into the basilar cells, much as described by Renson. They compare the intercellular substance of the spermatogemma to the blastophor described by Blomfield in Worms, and regard the follicular cells as a secondary addition peculiar to Vertebrates, and homologous with the follicular »cells of their ova.
The first step towards any understanding of the process of spermatogenesis amid this maze of controversy is to collate the various observations; hence the present attempt to summarize the main observations on the subject, and afford a key to the nomenclature. Brat how shall we reconcile the different theories? Each author formulates his own view of spermatogenesis, and sometimes even admits only a single method (e.g., Von Ebner, Blomfield, &c.); yet, unless we attach considerably greater weight to the observations of at least a majority of all these workers than they sometimes incline to grant to those of each other, the literature and iconography of histology become of little worth. Since La Valette, however, most observers have admitted the existence of several methods; the homology—indeed the primordial identity of the primitive germinal cells—of male ovule with female ovule has been often pointed out, and the general resemblance of the process of spermatogenesis to that of segmentation has been noted, and even accented by the use of terms like sperm-morula, sperm-blastula, &c. But it is not enough to make such a comparison in general terms; it must be either susceptible of definite refutation or confirmation by appeal to details; and the present writer has hence elsewhere attempted to compare the various modes of spermatogenesis with the corresponding modes of segmentation of the ovum, and so reconcile the conflict of testimony and opinion upon the subject. (8)
Ontogeny of the Spermatozoon. —The origin of the spermatozoon from the spermatocyte is, as we have seen above, a phenomenon of considerable complexity. It has long been known to arise from the nucleus and the tail from the protoplasm, and Flemming in 1880 gave further precision to our knowledge by tracing the head of the spermatozoon of Salamandra from the chromatin of the nucleus. Various observers had also noted, besides the nucleus, the appearance of a small denser mass of protoplasm within the spermatocyte, the “accessory corpuscle” ( Nebenkcrn , corpuscle ’ précurseur ), but its origin and fate have scarcely yet been settled with complete clearness. It appears, however, to arise from the nucleus, the remaining portion of the nucleus going to form the main portion of the head, while a film of superjacent protoplasm stretches over it, thickens somewhat to form the middle piece, and becomes drawn out into the filamentous tail, which frees itself from the remains of its spermatocyte and swims away (3 to 8 in fig. 5). Von Brunn has recently described in Birds the origin of this accessory corpuscle with especial clearness by the division of the nucleus of the developing spermatozoon. In Mammalia the “cap” of the young spermatozoon is described by Renson as nucleus, and by others as protoplasm, and it has been compared by many authors to the polar vesicle of the ovum. In Plagiostome Fishes Semper describes, and others confirm, the existence of an additional nucleus, or “problematic body,” which appears to correspond to the sum of the accessory corpuscles of all the spermatozoa, and this, after their escape, has been shown by Swaen and Masquelin to fuse with the nucleus of the basilar cell. The resemblance of this embryonic and transitory structure to the permanent post-nuclear segment of the curious almost amoeboid spermatozoon of Ascaris is closely suggestive of their homology, and it is also interesting to note, in figures of the developing spermatozoa of Selachians, their considerable resemblance to the fringed adult spermatozoa of many Platyhel-minthes. (8)
The Ovum,. History. —From the earliest times naturalists have of course been familiar with the form and function of the ova of a great number of animals; the only seriously disputable question (excepting that of spontaneous generation) has been that of the like origin of the embryo in Mammals. The history of the discovery is a curious one. Galen had described the human ovaries as testes muliebres, but the term ovary is due to Steno (1664), who like our most modern investigators started in his comparisons from the corresponding organs in Sharks and Rays. In 1672 Regner de Graaf, in a remarkable work upon the structure of the ovary and its accessory organs, not only described in the ovary in Birds and Mammals the follicles which now bear his name, and which he regarded as the ova, but made the generalization of the universal occurrence of ova throughout the animal kingdom, and even observed the ovum in the oviduct of the Rabbit. His opinion, however, was overborne by the authority of his more famous countryman Leeuwenhoek, who regarded not the oviduct but the ovary, and more precisely the corpus luteum , as the seat of development of the embryo. The observation of De Graaf was repeated at the end of last century by Cruikshank, and again by Prévost and Dumas. The definite establishment of our present knowledge of the Mammalian ovum dates only from 1827, when Von Baer clearly traced the ovum from the uterus back to its earliest appearance in the oviduct, and thence to its origin within the Graafian follicle. Piirkinje had meanwhile (1825) described the “germinal vesicle ” in the Chick, Coste showed its occurrence in the Mammalian ovum, and Wagner discovered (1836) the “germinal spot” in Mammalian and other ova. (9)
Mass and Chemical Composition of the Ovum. —The absence of any extensive measurements of the ovum in the various groups, either absolutely or in comparison with the bulk of the parent, renders it impossible to generalize with any great degree of definiteness. While from some comparisons it is at once apparent that the higher the organization the larger the ovum, many independently variable factors affect this, e.g., the size, organization, or maturity of the parent, the duration of stay in the oviduct, and the climate and other surroundings. Examples of such variation are readily seen; it is natural that self-supporting larval forms should start with smaller food-yolk than those which develop before leaving the egg, and among placental Mammals that the continuous supply of nutriment from the parental resources should.
supersede wliat would otherwise require to be a gigantic store of yolk.
Our as yet crude analyses of ova of course reveal the presence of numerous highly nutritive substances, both albuminous and fatty—none, however, characteristic of the ovum alone. The general analysis of the Fowl’s yolk is quoted by Hensen as follows:—water 472־ percent., albumin stuffs 156־, ethereal extracts 314־, alcoholic extract 48־, ash 1 = 100,—these results of course covering au unknown degree of complexity. Thus numerous more or less distinct albuminoid substances have been described by different authors—vitellin, ichthin, ichthidin, emydin, Ac., while nuclein seems to be of special importance; the ethereal extract yields very distinct fats, not only non-nitrogenous, but nitrogenous (e.g., lecithin); cholesterin and many other complex products of anabolic and kata-bolic change are also present; and so on. The subject is at present peculiarly unfit for profitable summarization, and details must be sought from the bibliography. (10)
Oogenesis .—Since the different modes of origin of the ova in the various groups are at once less complicated and in general outline less debateable than the corresponding process of spermatogenesis, in regard to which the state of opinion is so unsettled, a much less detailed account must here suffice, especially since a review of the different processes in any detail would involve an unwieldy compression of the available summaries of Balfour and others. Only a few of the more interesting and suggestive examples can be here mentioned.
The origin of the ova within the mesoderm of the Sponge, which has been already referred to, is obviously one of the simplest cases, recalling the occurrence of reproductive cells in such a sub-Metazoan form as Volvox. In regard to the Ccelentera the controversy over Van Beneden’s hypothesis of the endodermic origin of the ova has been already mentioned; and in connexion with this Weismann’s theory may be noted, according to which in most Hydroids the reproductive cells are only differentiated late in life, so that the actual germinal substance or mother-protoplasm is not present from the first in cellular form but in molecule groups, scattered among the somatic cells and spread perhaps over various asexual generations, to be gathered up at some favourable epoch and in the most convenient situation in the definite form of ova.
In the group of Vermes the general history is that some favourably situated cells of the epithelial lining of the coelom proliferate, enlarge, assume the characters of ova, and fall off into the body-cavity. A simple instance of this in Tomopteris has been already referred to (fig. 1).
Sagitta furnishes a very suggestive illustration of oogenesis—where one half of the primitive reproductive cell goes to form the ovary and the other half the testis. In Bonellia we find a beautiful example of the very frequent subordination of several reproductive cells to the perfecting of one. In each mass of possible ova (which arise from the division of primitive germinal cells situated round the ventral vessel just above the nerve cord) only one—adjoining the stalk and therefore near the source of nutrition—becomes a differentiated ovum, while the others atrophy. In Platyhelminthes the later stages of oogenesis are especially interesting, because the nutritive equipment of the ovarian ova is in many cases partially furnished by the direct absorption of some of the cells of the yolk-gland or vitellarium, which is itself probably a degenerate portion of the ovarian tract in which over-nutrition has checked reproduction. Thus the ovum comes to be the result of a number of cells. Weismann’s interesting observations on the winter-eggs of some Crustaceans afford beautiful illustration of the subordination of a large number of germinal cells to the nutrition of a few, while in Mollusca this nutrition of the ovum is otherwise effected by the direct passage of food-material from the ordinary epithelial cells of the ovarian pouches.
The oogenesis of Insects, which has been the subject of so much discussion, is chiefly characterized by the very frequent presence in the ovarian tube of a large number of germinal cells which are wholly nutritive, and which serve to equip the minority of truly reproductive cells. Such cases inevitably raise the long-standing dispute as to the unicellular character of the ovum,—some authors (e.g., Brandt) describing the ovum as a cell of the second order, formed from a complex of the primitive nuclei or “ooblasts," which are combined in the terminal chamber of the ovarian tube, and which unite to form on the one hand the germinal vesicle of the ovum with enveloping protoplasm, and on the other the surrounding follicular cells. One of the most recent discussions of Insect oogenesis is that of Will, who maintains the origin of the follicular nuclei from a multiple division of the original primitive nucleus or ooblast, the residue forming the germinal vesicle, a mode of origin previously maintained by other investigators both of oogenesis and spermatogenesis (fig. 7). In some Echinodermuta, and in other groups, it lias been repeatedly observed that the undifferentiated germinal cells form the elements of the follicular epithelium round the ova.
A debateable but suggestive theory of oogenesis has been propounded, especially by Kussbaum and Weismann, who maintain that in many cases the reproductive cells do not arise as differentiations of somatic cells, but are marked off from the first, in some cases even before the formation of the germ-layers. According to this ingenious hypothesis,—supported, however, by little direct observation—the reproductive elements would form a continuous immortal chain connecting the highest forms with their Protozoan ancestry, whose direct protoplasmic continuity has been of late also strongly insisted upon (see Puotozoa).'
Dispute has also arisen as to the origin of the follicular cells of the Tunicata, some deriving them from external juxtaposition of germinal cells, and others from migration of nuclei from within outwards, as in some Insecta ; and a similar uncertainty prevails as to the nature and origin of certain cell-like bodies (“test-cells”) which appear within the yolk.
In the Craniata the ova appear in the germinal epithelium of the ovarian ridge, which is always in contact with .the stroma. They differ from the surrounding cells at first mainly in their greater size, being possessed of abundant protoplasm and a large granular nucleus. Increasing in number by division or by continued differentiation of other epithelial cells, the primitive ova usually form into masses, as the result of which some atrophy and others predominate. The permanent ova once formed and defined are surrounded by a special follicle, probably resulting in most cases from superjacent epithelial cells.
None of the disputes above referred to can be said to invalidate the general view of the essentially unicellular nature of the ovum (see below); and a discussion of the numerous speculations on the more fundamental problem of the stages of sexual differentiation is deferred to the article Sex. (11)
Structure of the Ovum .—While the structure of a sufficiently young ovum is simply that of an ordinary embryonic cell, its protoplasm being naked and often amoeboid, and provided with a nucleus and nucleolus, the developed ovum has usually a quite characteristic appearance. Not only have its parts usually undergone considerable enlargement in size, but also in details of minute structure; the nucleus and nucleolus are, however, still recognizable as the germinal vesicle and germinal spot, while the protoplasm has usually become modified by the presence of a more or less considerable quantity of food-yolk, and by the development of an external membrane.
The young amoeboid phase of so many (perhaps all) ova was first well described in the egg of Hydra (fig. 8, h), in which the nucleus and nucleolus, the lobed pseudopodial processes of the protoplasm, and the abundant yolk spherules can be well made out. A somewhat later phase of development is w^ell shown in the next figure of the egg of a Sea-Urchin, in which the process of encystment has begun, and the protoplasm is seen with its amoeboid processes radiating through the incipient egg-membrane, while in fig. 8, d the protoplasm has no longer an amoeboid character—the investing membrane or zona radiata is regularly perforated by radiating canals, of which the pseudopodial origin and the nutritive function are equally obvious. The resemblance of such stages to definite Protozoan types is not a little remarkable; thus, while, in fig. 8, b is perfectly amoeboid, the resemblance of c to a Heliozoon or of d to a Gregarine is almost equally striking.
The ovum is rarely destitute of egg-membranes, but these may be of very various kinds: thus we may have a vitelline membrane proper, formed by the protoplasm of the ovum, or a chorion formed by the cells of the follicle; or secondary membranes may be present (alone or in addition to the primary membrane) like the shell of a Bird’s egg, which is formed by the walls of the oviduct, or the shell of many Trematode ova, which is secreted by a special gland.
Any or all of these membranes may be provided with a special aperture, the micropyle, first discovered by Keber, but this is by no means universally present, as he supposed. This may correspond to the point of attachment of the immature ovum, or may arise elsewhere; in the first case its function is obviously nutritive, though later it may also serve for the entrance of the spermatozoon.
The identification of the ovum as a cell, and of its germinal vesicle and spot as nucleus and nucleolus, although a result only established after prolonged controversy, and of capital importance, is sufficiently familiar. The invaluable labours of the older generation of embryologists from Yon Baer to Allen Thomson, and even the still classical monographs of sucli recent workers as Waldeyer or Ludwig (1874), can only be alluded to; this department of the subject is, however, of peculiarly easy access, thanks to the exceptionally excellent state of its bibliography, and to the recent discussions of Balfour, Hensen, and others. Much, however, remains to be ascertained respecting the finer histology of the ovum, and many investigations are at present in active progress, along the lines of that more general inquiry into the minute structure of cells in general which has of recent years been again becoming of paramount interest in morphological research.
The protoplasm of the ovum may, as has been said, acquire a very varying quantity of food yolk,—may become, that is to say, more or less closely packed with highly refracting spherules of modified protoplasm (see egg of Hydra, fig. 8, b), which may again present various morphological differentiations, as in Ascaris (fig. 11). And, as explained below under segmentation, it is with regard to the presence, amount, and position of the food-yolk that the important varieties of that process are to be understood.
The great differentiation both of protoplasm and of yolk in the animal series, as might be expected, appears to exhibit all gradations from the most simple amoeboid state of a more or less granulated or semi-fluid mass to the most problematical complexity. Of this the best known instance is probably that of the egg of Ascaris, described by Van Beneden (vide infra, pp. 416 sq. ). Not only has a protoplasmic network been frequently described in both holoblastic and meroblastic ova, but a radiate structure as well,—the former evidently corresponding to the stroma first described by Frommann and Heitzmann and subsequently by so many authors in both animal and vegetable cells, while the latter appearance recalls the striated appearance of the ectoplasm of certain amoeboid organisms described by Strasburger. The concentric differentiation of the ovum has also often been described, and is lately well discussed by Flemming, Brass, and others. Thus Pflüger, in the half-ripe ovum of the Cat, describes the central mass as clear, the cortical layer as rich in granules; Yan Beneden in the egg of the Bat enumerates two similar layers surrounded by an almost granular cortical layer; while Flemming himself, in the ovum of the Rabbit, describes a coarsely granular region around the nucleus, a clearer central region finely granular and with a reticulated structure, and a coarsely granular cortical region. \Ve have still, however, to learn how far such differentiations in structure—reticulate, radial, and concentric—are constant for individual or general cases, and how far they may be permanent or merely incidental to certain phases of development.
In these regards the recent publications of Carnoy and Brass are of special interest—the former on account of its minuteness of micrographic detail, the latter in its attempt at physiological interpretation. The results of the former will be best understood from his own figures (fig. 9).
Brass, starting from the familiar structure of an amoeba, with its clear and granular ectoplasm, from which the pseudopodia are emitted, its semi-fluid and highly granular endoplasm around the nucleus, and the less granular intermediate zone, assigns to these definite physiological functions,—to the first that of contractility ( Bewegungs - plasma), to the second that of nutrition ( Emdhrungs - plasma), and to the third that mainly of respiration (Athmungsplasma, Nahrungsschicht). He holds that some such concentric disposition of the protoplasm is a normal and constant fact of cell-structure, and insists upon it with special reference to the ovum. From his somewhat vague and diffuse development of these views, it must suffice here to note his opinion that the chromatin of the nucleus, as well as the protoplasmic reticulum, and in fact all the former constituents of the cell, are of quite secondary importance to the colourless protoplasm. The former is to him in fact no more than reserve material, while to the latter he assigns all active functions,—thus” substantially reviving the view so long and energetically maintained by Beale. The recent observations of Wielowiejski may also be noted in this connexion, as he not only brings his contribution to the ontogeny of the ovum, but to some extent also distinguishes a regional adaptation of its protoplasmic structure-to its functions.
From the rapid succession of new contributions to the solution of the problem of egg and cell structure might be gathered many other points of interest, morphological and physiological, empirical and speculative. Thus, for instance, Sabatier describes senile degeneration in Ascidian ova, an observation of wide suggestiveness; while, again, on the important problem of the relation of nucleus to protoplasm there are many recent discussions, e.g., from Flemming and Strasburger, Pflüger and Hertwig; the solution, however, is not complete. Again, can we recognize in the ovum any indication of the position of the future embryo—any fixed points, anterior and posterior, lateral or even polar—further, of course, than the obvious distinction due to the presence of yolk? After the old theory of “evolution” of the embryo, according to which the egg contained the complete organism in miniature, had been finally replaced by that of epigenesis, the wholly undifferentiated form of the ovum seems to have become tacitly assumed. Recent observers, e.g ., Van Beneden (see below), have, however, been so far reviving the old view in that they endeavour to distinguish, even in the unfertilized ovum, the position of the ends and sides of the embryo; others dispute this, and an interesting controversy is in progress. The speculation that a more or less considerable share in the differentiation of the ovum might be due to the separation of its various constituents according to their different specific gravities—at first apparently emitted by Jäger,—has also reappeared in this regard. Pflüger has observed the segmentation of Frogs’ ova fixed in various positions, and describes the plane of first segmentation as constantly vertical, whatever might be its angle to the morphological axis of the ovum, uniting black and white poles. Development too was usually normal, save that when the upper hemisphere was entirely white abnormality and death followed, and even if inversion was less complete segmentation often stopped. Further experiments led him to the conclusion that the nervous system, and correspondingly other organs, may develop from any portion of the egg-substance—that the egg in short is “isotropic.” Certain limitations, however, appeared: the blastopore never arose on the upper hemisphere, nor (like the nervous system) ever on the black region, but both always at the intersection of the white area with the third equatorial plane of segmentation. This he terms the point of crystallization of the specialized organism, and goes on to speculate as to the molecular structure of the ovum.
These results have, however, been the subject of keen criticism. Thus Roux showed that the specific gravity of the black pole is distinctly less than that of the white, and found that on eliminating the action of gravity by the use of a centrifugal machine the development remained normal, and on repeating Pfliiger’s own fundamental observations described the axis of segmentation as coinciding with that of the ovum. After similarly eliminating light, heat, and earth-magnetism, he sums .up in precisely opposite terms to Pflüger, who had laid all stress on the influence of the forces of the environment, and regards development as purely a process of self-differentiation. O. Hertwig also maintains that the influence of gravity is only a secondary one, the plane of division being for him determined by the position of the axis of the dividing nucleus, and this again having a definite relation to the form and state of differentiation of the surrounding protoplasm. He admits, however, the indirect importance of gravity on eggs having a yolk, and so leaves the question still to a considerable extent open. In another paper he adopts Pflügers conception of the isotropy of the ovum, holding that the yolk is not so organized that from any definite region of it a definite organ arises, but that the nucleus is the sole centre of activity and control. It is thus evident that a reinvestigation is needed which would embrace the whole question of geotropism. See Physiology (Vegetable). (12)
Maturation of the Ovum.—Polar Bodies .—The period of development and nutrition of the ovum may be regarded as complete when the full size and complex structure above described have been reached, and in this state it most frequently leaves the body of the parent. In the majority of cases at least, new changes have still to be gone through before fertilization takes place, still more development; and a series of important structural modifications, doubtless the expression of extensive functional rearrangements, takes place. To this new and obscure phase of the life-history of the ovum the term maturation has conveniently been applied.
Although some of these phenomena have long been familiar to embryologists, the classical investigations are the comparatively recent ones of Biitschli, Oscar Hertwig, and especially Fol (1877); more lately those of Sabatier, of Flemming, and above all of Van Beneden. If we postpone details, the main facts of the process as until recently understood can be most readily grasped from Fol’s figures of the ovum of the Starfish, of which the most important are copied in fig. 10. In the new-laid egg the germinal vesicle is at first a clearly defined sphere, with well-marked membrane, reticulum, and germinal spot. It begins, however, to become irregular and changeful in form, its membrane and reticulum meanwhile disappearing, with the apparent dissolution of a portion of its contents in the yolk. The germinal spot also vanishes, and two ill-defined clear spots are alone distinguishable in the yolk. Treatment with reagents shows these clear spots to correspond to two star-like figures like those of a certain stage of the division of nucleus of an ordinary cell, while in a somewhat later stage a nuclear spindle is shown near the surface of the egg. The upper portion of this is segmented off (surrounded by very little of the protoplasm of the ovum) as a polar body, while the lower portion of the body remains as the “female pronucleus,” and may be seen surrounded by radial striae. Two or even more polar bodies may be formed, but they never take any part in the subsequent life-history of the ovum, and sooner or later disappear.
In view of the profound theoretic importance of this subject, it is necessary to summarize some of the most important recent observations. Thus Balbiani and Weismann (1882) describe the occurrence of polar vesicles in the ova of Insects, in which they had long been supposed not to occur, and Flemming (1885) represents a nuclear figure which he regards as corresponding to at any rate the first stage of the formation of a polar vesicle, in the ova of Mammals, at the period of bursting of the Graafian follicle. Such researches practically complete the chain of evidence for the generalization that the process of maturation of the ovum is essentially similar in all classes of Metazoa, and this not only in its initial stages of internal rearrangement or modification, but even in its later stage of polar vesicle formation. The absence of polar vesicles in parthenogenetic ova (predicted on theoretic grounds) seems, however, possible in some cases, but is by no means certain in all.
Our knowledge of the process of maturation has been much extended and modified by the recent elaborate monograph of E. Van Beneden (1883). He finds the egg of Ascaris to show not only two definite poles—a “pole of impregnation” and an opposite “ neutral pole ”—but some indication of the anterior and posterior regions and consequently of the right and left sides of the embryo. The substance of the ovum is also remarkably differentiated,—that of the “polar disk” alone exhibiting a vertical striation, and differentiating into two layers, superficial and subjacent (termed achromophilous and cliromophilous respectively). The subjacent vitellus is distinguished into several layers, (central, intermediate, and cortical), and contains yolk elements of three distinct kinds, while the enveloping protoplasm has a distinctly reticulated and !fibrillar structure, like the stroma of so many animal and vegetable cells. The germinal vesicle is practically homogeneous save that the portion surrounding its eccentric germinal spot is distinguished into an investing portion or spherical envelope, the “prothyalo-soma,” and an “accessory portion,” which usually contains one or two “pseudo-nucleoli.” In the ripe egg Van Beneden describes new complexities within the germinal spot; this consists of two juxtaposed quadrilateral disks, each containing four chromatin globules, united by a substance having less affinity for colouring matter. lfadiating from these two disks, achromatin threads arise in the prothyalosoma, but stress is laid on the fact that no grouping of the chromatin elements into a stardike figure takes place. The spherical shape of the germinal spot is now modified by the intrusion on each side of a large homogeneous droplet from the vitellus into the prothyalosoma, so that in optical section it comes to have a T-shape, the accessory portion being mainly compressed to form the stalk of the T. At this stage the spermatozoon usually commences to work its way into the ovum, but does not yet affect the germinal vesicle or germinal spot, which proceed to the formation of polar globules. The T-shaped gradually passes into the “ypsiliform” figure, so called from its resemblance to the Greek T. Its steadily diverging branches, which are formed from the prothyalosoma, move upwards till they reach the surface of the vitellus, their fibrillar structure already noted meanwhile becoming well marked. Each bundle bears one of the two groups of four chromatin globules which compose the germinal spot.
Next the vertical branch of the ypsiliform figure swings upwards to the surface, and a new branch is formed as a continuation of the same line; the whole figure is thus cross-shaped, with the prothyalosoma in the centre, but this cross soon disappears, leaving the prothyalosoma with its two chromatin groups. These are now divided, by a plane tangential to the surface of the vitellus, into two equal parts, and the upper of these, containing of course half the prothyalosoma with half of each of the two chromatin disks, becomes the first polar globule. Yan Beneden lays great stress on the fact that this plane of division is not transverse to the oblique spindle formed by the diverging branches of the T, as was to be expected from the views of all previous observers, but on the contrary is parallel to it.
The remaining portion of the prothyalosoma with its two chromatin disks, together with the surrounding protoplasm, now proceeds to the separation of the second polar vesicle. Despite differences in detail, the essential facts are the same: a spindle with a star-like figure at each end is formed; this at first lies radially in the vitellus, but afterwards becomes superficial, and its division takes place as before by an axial not an equatorial plane.
A still later and also elaborate discussion is due to Sabatier (1884). While combating the peculiar views suggested by
Weismann and others as to the polar globules of Insects (as that they re-enter the ovum to form the rudiments of the future reproductive organs), he admits that they may be emitted at both poles of the ovum, and may either break up, be reabsorbed, or even in some cases form peculiar structures surrounding the ovum (e. g., the follicular cells of Ascidians). He describes in Buccinum, &c., the extrusion of somewhat amoeboid masses of clear protoplasm at several distinct points and frequently without the appearance of any nuclear spindle; and these may even repeatedly divide. He holds that the centrifugal extrusion of elements from ova is much more general than is usually recognized, and distinguishes these into three kinds :—(1)
“globules précoces,” which usually go to form the elements of the follicle; (2) “globules tardifs,” which appear later; and (3) true polar vesicles, which alone are associated with karyokinetic changes of the nucleus.
Like Yan Beneden, he notes that polar vesicle formation is not strictly comparable to ordinary cell-division in being not transverse but longitudinal to the nuclear spindle.
From this point little advance has been made, though careful reviews of the subject are due to Flemming, J. T. Cunningham, and others. (13)
Fertilization or Impregnation of the Ovum .—The fundamental generalization now so familiar—that the process of fertilization for plant and animal alike lies in the material union of both sexual products—although said to have been propounded by Alcmaeon (580 b.c. ), and even described by Hartsoeker (1750), is pointed out by Hensen to be essentially due in the first place to the experimental researches on plant hybridization of Kolreuter (1761). Jacobi soon afterwards artificially fertilized the eggs of Trout and Salmon, but the most important and really convincing work was that of Spallanzani (1780), who experimented on the Frog, Tortoise, and Bitch. He unfortunately, however, concluded that spermatozoa might be absent without preventing fertilization, and ascribed all fertilizing powers to their fluid medium, so establishing an error which required for its elimination many successive researches. Prévost and Dumas (1824) showed that filtration really deprived the seminal fluid of its powers. Martin Barry (1843) actually observed spermatozoa within the zona pellucida of the Rabbit’s ovum; Leuckart (1849) repeated both preceding observations in the Frog; Nelson (1852) observed the entrance of the spermatozoa into the ovum of Ascaris ; while Keber (1853) discovered the micropyle in the ovum of the Mussel, and watched the passage of the spermatozoon through it into the yolk. It only remained for these results to be confirmed by the high authority of Bischoff and Allen Thomson (1854); and the knowledge of the subject thus reached its second stage, where it practically remained for nearly twenty years.
Aided by the advance of histological technique, a new plane was reached (1875-76) by the brilliant researches of Yan Beneden on the ovum of the Rabbit, and of O. Hertwig and Fol on Echinoderm ova. The spermatozoon was thus shown not to disappear into the yolk, but to form from its head or nucleus the “male pronucleus,” which meets and fuses with the “female pronucleus,” as the germinal vesicle is termed, after the extrusion of the polar body, This new “conjugation-nucleus” soon exhibits karyokinetic changes and divides, and the segmentation of the ovum rapidly progresses.
As a full account of these and other papers up to 1880 is given by Balfour ( Embryology , vol. i. ), it will suffice to note the more important subsequent researches, especially as those, though with one exception confirmatory, penetrate somewhat further into details. The general process can be made out with especial clearness in fig. 13.
Oscar Hertwig had clearly enunciated in 1875 that “fertilization depends upon the conjugation of two sexually differentiated nuclei.” This view has, however, lately (1883) been controverted by A. Schneider, who regards the radii which are seen in the ovum about the pronuclei as arising from the substance of the germinal vesicle alone, and who maintains the origin of the male pronucleus independently of the spermatozoon, which he believes to disappear. As Van Beneden, Nussbaum, and Flemming have since, however, successively controverted these views, Hertwig’s account of the process may safely be retained, at any rate with the slight modification insisted on by Flemming, who prefers to describe the conjugation-nucleus as arising from the union of “the chromatin of a male with that of a female nuclear body.”
In Yan Beneden’s work on the fecundation of Ascaris the penetration of a single spermatozoon into the vitellus and its transmutation into the male pronucleus are copiously figured and described. Most important, however, is perhaps his account of the formation and segmentation of the conjugation-nucleus. The. female pronucleus undergoes changes resembling those preceding ordinary nuclear division; so also does the male; and the chromatin of each breaks up from the state of a single continuous and convoluted fibril into two V-shaped loops. The two pro-nuclear membranes now fuse; and the resultant conjugation-nucleus thus contains four loops of chromatin, of which two are male and two female. Each loop now undergoes longitudinal division; a nuclear spindle is meanwhile forming from the achromatin substance of the segmentation-nucleus, and this now proceeds to divide, but in such wise that each of the two daughter nuclei receives two of its four half loops of chromatin from the female and two from the male pronucleus. A circular furrow appears, dividing the ovum equatorially; a cell plate like that of a vegetable cell arises in the same plane; and the two first blastomeres are soon completely formed, and proceed to redivide in the same way.
The results of Nussbaum are essentially similar so far as they go; Strasburger’s recent work summarized below contains a thorough confirmation of these accounts of the process of fecundation so far as plants are concerned; while most recently (1885) 0. Hertwig has re-stated his original theorem with a discussion of its physiological aspects and consequences. (14)
Segmentation of the Ovum .—The process of segmentation has already been repeatedly mentioned. As with the other histological problems presented by the ovum, our knowledge of its minute details is in a somewhat unsettled yet rapidly advancing state, due to the progress of the wider inquiry into cell structure and cell division in general. The essential correspondence of the changes to be observed in the conjugation-nucleus and yolk with those of ordinary cell-division (see Protozoa, vol. xix. p. 833, fig. 1) has been worked out by many observers, and Van Beneden’s more minute account of the process is summarized above. Our knowledge of its external features is of much older date, and has been comparatively recently brought together by Balfour ( Embryology ., i., chap, iii.); the present limits permit only the briefest summary.
In the simplest case—that of small ova destitute of food-yolk,—the ovum divides by a vertical plane into two segments or blastomeres, each of these again into two, and so on, and a mass of 4, 8, 16, 32, 64, &c., cells is thus formed, but in the resultant mulberry-mass or morula the cells are all equal and similar. Such segmentation is termed regular; commonly, however, the regularity and equality of ordinary cell-division is more or less interfered with by the presence of food-yolk. In this regard Balfour enunciates the valuable general law “that the velocity of segmentation in any part of the ovum is, roughly speaking, proportional to the concentration of the protoplasm there, and that the size of the segments is inversely proportional to the concentration of the protoplasm.” Bearing this in mind, the varieties of segmentation are intelligible enough; thus the unequal segmentation of the Frog’s ovum (fig. 15) needs no further explanation. These two preceding types, in which segmentation is complete, were distinguished by Remak as “holoblastic,” and were long looked upon as wholly distinct from “meroblastic” ova like that of Fowls or most Fishes, in which segmentation is partial, i.e., confined to a small area of the surface of the yolk.
Balfour’s law, however, explains these cases as only an exaggeration of the preceding case, due to the greater accumulation of food-yolk, and the consequent check to segmentation at the lower or vegetative pole. Another form of partial segmentation is presented by many Arthropods, in which the yolk occupies a central position within the ovum, and three varieties of this “centrolecithal” process are distinguished—equal, unequal, and superficial.
While the ontogenetic process of segmentation in general and of regular segmentation in particular is usually regarded as being a recapitulation of the phylogenetic development of the primitive Metazoon from its Protozoan ancestor, the vast nutritive importance of the food-yolk, and the consequent high variability of its quantity in relation to the habits and circumstances both of parent and offspring, render its wide variations in detail a totally unsafe guide to affinities. A single genus may, in fact, exhibit extreme forms. (11)
§ 3. SYSTEMATIC AND GENERAL EMBRYOLOGY.
While it is impossible to compress within the limits of the present article the subject-matter of a full treatise on embryology, an attempt must be made to touch in turn upon (1) the empirical facts, (2) the concrete inductions, and (3) the abstract generalizations of development.
1. A sketch of the history of this branch of the science is given under Embryology ( q.v .), while for detailed information as to the division and differentiation of the ovum in each group, and as to the more or less marked changes by which the resulting embryonic forms assume the adult organization, the reader must be referred to the separate articles (Sponges, Hydrozoa, Mollusca, &c.), as w׳ell as to the classical work of Balfour. The more important empirical facts need thus only the briefest outline.
Protozoa.—Since the young Protozoon, whatever its mode of origin from the parent form may be, rarely differs from the latter except in size and phase of cell-life, there is little to be noted here in regard to the development of the group, except in such cases as Katallacta and Volvocineaz, where a primitive attempt towards a more or less temporary multicellular form affords us a first hint of the Metazoan segmentation of the ovum.
Dicyemida. —In this degenerate and still somewhat problematical intermediate group the asexually produced ovum segments to form an embryo of great simplicity, where a few outer (ectodermal) cells cluster round a single inner (endodermal) one, and where we thus find the first differentiation of the embryo into the cell-layers constantly exhibited in the succeeding groups, though it is unsettled whether this represent the incipient or the degenerate Metazoan type.
Porifcra. —In a calcareous Sponge, such as Sycandra, the fertilized amoeboid ovum segments within the parent to form a hollow blastosphere, most of the cells of which are clear and ciliated, and partially cover a number of larger granular cells,—the future endoderm and ectoderm respectively. In the free-swimming embryo the latter nutritive cells increase rapidly in size, and, growing ultimately over the locomotor and respiratory ciliated cells, cause them to become invaginated to form the endoderm. This invagination resulting in the obliteration of the “segmentation cavity” and the formation of the “gastrula” stage is perfected as the young Sponge settles down and becomes fixed by processes from the amoeboid ectoderm cells. The cilia of the endoderm disappear; the blastopore is obliterated; between ectoderm and endoderm there appears a third germinal layer or mesoderm, and in it the calcareous spicules of the Sponge are developed. A perforation at the top of the inverted gastrula forms the exhalent aperture; peripheral inhalent pores also arise; and this renewed exposure of the endoderm cells to direct contact with the water is followed by their return to the ciliated form. See Sponges. Ccelentera. —The generally equal segmentation of the ovum results as usual in a solid or hollow morula. The formation of the two layers, as in some other groups, may occur either by invagination, as above described, or by a process of separation of inner from outer, termed delamination. In most Ccelentera the tissues which in higher groups are strictly mesodermal originate from the direct differentiation of the two primary layers. The larval form most constant in the Ccelentera is the planula (see Hydrozoa, vol. xii. p. 557). It is ciliated, two-layered, almost cylindrical, witharudimentary digestive cavity, generally closed. For discussion of the alternation of generations so characteristic of many Coelenterates see Hydrozoa and Sex. Vermes. — The development of the unjointed Worms is too varied to admit of general summary. In some we find illustration of that mode of gastrula formation (epibolic), so common in higher groups, where, owing to the large size of the food-packed endoderm cells, an actual invagination is impossible, but the ectoderm cells grow round the others and thus come to enclose them as in the more primitive process. In such a typical higher Worm as Lumbricus, the segmentation, which varies in regularity according to the species, results in the formation of a blastosphere with readily distinguishable ectoderm and endoderm; the invagination of the latter results in a somewhat cylindrical gastrula. The blastopore narrows anteriorly to form the permanent mouth, while in some other Chsetopods a similar narrowing in the opposite direction forms not the mouth but the anus. The mesoderm appears very early in the form of posteriorly situated cells on each side of the middle line, and soon two mesoblastic bands are formed extending to the mouth. These are subsequently divided into blocks or somites, the anterior and posterior walls of two adjacent blocks uniting to form the cross partitions of the adult Worm, while the outer and inner walls form respectively the somatic and splanchnic layers. The ventral nerve cord results from the sinking in and union of two epiblastic thickenings, developed one on each side of a ventral furrow, while the supra-cesophageal ganglion has a distinct origin from a dorsal thickening. The somewhat isolated Sagltta affords good illustration of the formation of the body-cavity from two lateral lobes of the archenteron. The simple enteric cavity normally formed exhibits anteriorly a division into a middle lobe and two side pouches uniting behind in the single cavity. The middle lobe is separated off from lateral and posterior cavities as a blind tube which afterwards forms the alimentary canal; the side pouches form the body-cavity (see Tapeworms, Planarians, Nemertines, Annelida, &c.).
Echinodermata. —The generally uniform segmentation results in a blastosphere, usually elongated in the direction of the axis of invagination. A gastrula is formed by normal invagination, in the course of which amoeboid cells are budded off from the two sides of the advancing depression to form the mesodermic musculature and connective tissue of the adult. The blastopore forms the larval anus, which does not always persist; the larval mouth is permanent, and is the result of an independent ectodermic invagination meeting the archenteron. The coelom or body-cavity, with its mesodermic lining, arises from a paired or single outgrowth of the archenteron, and the water-vascular vesicle which comes to lie round the oesophagus has a similar origin. For an outline of the nature of the larval forms and the passage of the embryonic organs into those of the adult the reader is referred to Echinodermata. Arthropoda. —The interesting Pcripatus (see Myriapoda, vol. xvii. p. 116) exhibits in its development phenomena, such as that of the elongated blastopore gradually closing from the middle, so as to leave mouth and anus, and that of the surrounding nerve cord, which suggest affinities through the whole animal kingdom from Ccelentera on the one hand to Vertebrata on the other; but of these discussion is still far from ended. The development of the Insect ovum illustrates what is termed centrolecithal segmentation, where a single uniform layer of superficial cells encloses a nucleated but undivided central yolk mass. The blastoderm becomes thickened ventrally; and along the middle line of the ventral plate there appears a groove which is the seat of the mesoderm formation. The development of the Vertebrate amnion is recalled by the formation of two enveloping membranes from folds of the blastoderm, which arch over and coalesce over the ventral plate, and at the same time spread upwards to cover the dorsal integument from which they are eventually separated. Both disappear through absorption or rupture either before or during hatching. The mesoderm is divided into two lateral bands, and these into hollow somites, which together form the perivisceral cavity. As usual the outer layer of mesoderm follows the ectoderm, the inner the endoderm, which probably arises from a secondary segmentation of the yolk. The ventral plate extends to a slight extent dorsally both before and behind, and on it the appendages appear as hollow outgrowths of ectoderm and endoderm. The ventral nerve cord arises from two sunken thickenings of ectoderm, the tracheae as invaginations of the same. The middle section of the alimentary tract is formed from endoderm, the anterior and posterior portions from ectodermic invaginations. For an account of larval metamorphosis, &c., see Insects. Crustacea. —In these the segmentation is usually centrolecithal, and results in a uniform blastoderm enclosing a central yolk mass without traces of division. The gastrula stage is represented by an invagination of a patch of the blastoderm, which forms the endoderm, and from which the mesodermic cells seem also to arise. A ventral thickening of ectoderm continuous with the invaginated portion forms the ventral plate on which the embryo is mapped out. Along it the nerve cord appears as a median thickening, and the sense organs have as usual a similar ectodermic origin. The mesoderm appears to form the usual tissues (muscles, heart, &c.), but has apparently less definiteness of arrangement into layers and somites than is usually the case. The original invagination forms the mesenteron, and the lining endoderm cells absorb the yolk The anterior and posterior portions of the alimentary tract are formed as usual by subsequent independent invaginations. Much less proportionally is known of the organogeny than of the very interesting and suggestive larval forms, for which see Crustacea. Mollusca. —The typical Molluscan segmentation is markedly unequal, the disproportion between ectoderm and endoderm cells varying according to the proportion of food-yolk. The gastrula stage is more frequently reached by the growth of the ectoderm cells over the larger and fewer endoderm cells than by any actual invagination. The blastopore may close apparently towards either the oral or the anal extremity, forming mouth or anus or neither. In Cephalopoda the segmentation is confined to a germinal disk of formative material as in partially segmenting Vertebrate ova, though there are besides cells from the yolk which underlie the germinal region of the blastoderm and also accompany it in.its growth round the yolk, forming the inner blastodermic layer or yolk membrane. The mesoderm appears at the lips of the blastopore, or in an equivalent position; it forms a complete layer between ectoderm and endoderm, and within it the body cavity is formed usually by a definite splitting into two layers. The invaginated endoderm forms the mesenteron; and the history of the formation of the anterior and posterior portions of the tract varies with the fate of the blastopore. In the Cephalopoda at least, the nervous system appears not as usual from the ectoderm but from the mesoderm. The characteristic larval organs, the shell gland and the foot, are ectodermic in origin—the former arising as a thickened invagination on the posterior and dorsal side of the embryo, and the latter as a ventral protuberance. Further details and larval changes are thoroughly treated under Mollusca. Tunicata. —The complete and generally quite equal segmentation results in the formation of a blastosphere with a large segmentation cavity. The cavity is obliterated by a normal invagination, and a typical gastrula is formed. In the formation of the nerve cord from the closure of a medullary groove and of the notochord from a cord of cells on the dorsal wall of the archenteron, in the appearance of respiratory slits in the anterior portion of the enteric cavity and of a cerebral eye, and in fact in all the essential features of their development they present the amplest evidence of their Vertebrate affinities and degenerate origin (see Tunicata).
Vcrtcbrata. —The segmentation is total and equal in Amphioxus, total and not markedly unequal in Mammalia, total and unequal in Amphibia, Acipenser, Pctromyzon, partial in Birds and Reptiles, Teleosteaus and Elasmobranchs, but the limits between these types are not sharply defined, being mainly determined by the store of food-yolk. In Amphioxus there is no interference with the formation of the typical gastrula, and this form is also readily recognizable in the Amphibia, Acipenser, and Pctromyzon, where, however, the process of formation is less simple. At a point corresponding to the future hiud end of the embryo a true invagination takes place, forming, however, only the dorsal wall of the archenteron, at the same time the ectoderm cells have been growing round the yolk, and the floor of the enteric cavity is formed from yolk-cells. The blastopore is eventually closed, but a communication between the archenteron and the neural canal persists for some time as the neurenteric canal. In the Elasmobranchs and Teleosteans, though there is no true invagination nor recognizable gastrula form, there are not wanting hints of its ancestral occurrence and of homologies with the typical form. The comparison of the different Ichthyop-sidau gastrula; with the typical form and with that of the Saurop-sida, and such problems as the occurrence of an epibolic gastrula in Mammalia are still under discussion. The most important stages in the further Vertebrate development, the formation of the medullary groove and canal, the appearance of the notochord as an axial differentiation of hypoblast, the origin of the body-cavity from paired outgrowths from the archenteron, the occurrence of gill-pouches opening externally from the throat, the expansion of the medullary cord anteriorly to form the vesicles of the brain, the outpushing of a cerebral eye, and so on, are described in all text-books. (II)
2. But it is not enougli empirically to record the observed phenomena of reproduction in the various groups, primarily essential though such labours are; it is further necessary to generalize them. We should ascertain, that is to say, from a comprehensive and a comparative survey, what degree of unity of structure is discovered actually to prevail within and among the various groups. Were this knowledge once definitely reached and incorporated with the results of comparative anatomy and palaeontology, that complete conspectus of the animal kingdom which is the goal to which all morphological inquiries converge would come clearly into view. Such an exposition of the contributions of the study of development to morphological knowledge may evidently proceed in either of two ways,—either by starting from the later and more superficial similarities which are expressed in the classification of the different groups, and working downwards towards earlier and deeper unities in the various organs and tissues; or by starting from the most primitive homology, and working upwards, so demonstrating the unities which are observed to obtain in the successive stages, from segmentation and layer differentiation onwards. For our present purpose, it is more convenient to follow the latter course, and to note the main concrete generalizations of development in ascending order from the deepest and most general to the more superficial and specific.
1. Ovum Theory. —The most fundamental unity upon which all the others really depend is the familiar fact that all organisms, plant and animal alike, develop from a single germinal cell or ovum essentially comparable with any ordinary cell. The importance of these results may be best expressed by a quotation from Agassiz’s once famous Essay on Classification :—Von Baer “thus showed for the first time that there is no essential difference in the mode of reproduction of the so-called viviparous and oviparous animals, and that Man himself developed in the same manner as animals. It was indeed a gigantic step to demonstrate such an identity in the material basis of the development of all animals, when their anatomical structure was already known to exhibit such radically different forms. The universal presence of eggs in all animals, and the unity of their structure, which was soon afterwards fully ascertained, constitute in my opinion the greatest discovery in the natural science of modern times.” The ovum has long ceased to be regarded as a mysterious microorganism, and, while in some cases a number of cells unite to nourish and perfect it, the generalization remains unshaken, that the ovum of every organism is a simple cell. The minute structure of the ovum, its differentiations and variations, its fertilization and development, must be expressible in terms of the morphology and physiology of the ordinary cell. The elucidation will be obviously mutual, for the analysis of the phenomena observed in the case of the highly differentiated reproductive cells, with their consequent greater physiological simplicity, will shed light on analogous phenomena in ordinary cellular and Protistau life where, with less differentiation, there is greater physiological complexity,— while such problems as the origin and import of polar globule extrusion, fertilization, &c., receive some elucidation from suggestive analogies among the Protozoa. See Sex. (11)
2. Segmentation. —The segmentation of the ovum of the Metazoa varies in regularity and completeness throughout the groups, but the different types are not sharply defined from one another and are, as has been already noted, partially at least explicable as mechanically conditioned by the quantity of nutritive as opposed to formative material. Segmentation being only a special case of ordinary cell-multiplication (to which in behaviour of nuclei, &c., it fully corresponds), it awaits whatever elucidation may arise out of the present conflict of speculation as to the physical and physiological causes of division in general. It has further to be compared on the one hand with such cases of multiple division as are observed in spermatogenesis, and on the other hand, and more especially, with the earliest attempts to form cell aggregates and so effect the passage from Protozoa to Metazoa. (11)
3. Gastrsea Theory. —We have seen in the development of the various groups how the result of segmentation is not a mere aggregate of cells but an integrated two-layered individual with a distinct enteric cavity. In 1872 Haeckel emphasized the importance of this form, and in 1875 he elaborated his “gastraea” theory, according to which all animals pass through a gastrula stage which, in its typical form, resembles the ancestral Mctazoon. He showed how the variously modified quasi-gastrula forms of the higher groups might be derived from the normal type, and corroborated his theory by reference to the persistence of the gastrula stage in certain Porifera, as well as in those Ccelentera and Vermes where the adult animal is not far above the level of the gastrula organization. The occurrence of the planula and the delamination process in some cases alongside of the invaginate gastrula is a difficulty in the way of his theory so far as it defines the primitive Metazoan ancestor, and this has given rise to Lankester’s rival planula theory. The case on either side is fairly stated in Balfour’s Embryology (vol. ii. chap. xiii.). (Recent researches on Hydrozoa seem to show that delamination is an extreme form of immigration, which is itself a modified invagination.) The passage from Protozoa to Metazoa was, according to Biitschli, effected neither by planulan nor gastrula but by a disk-like “placula.” The ingenious “coelom theory” of the Hertwigs is an attempt to divide all the Metazoa with three germinal layers into two distinct groups characterized by different modes of formation of body-cavity and mesoderm, and by other minor differences. In the Enterocoda, which include the Chxtopoda, Gephyrea, Brachiopoda, Nematoda, Arthropoda,Echinodermala, Enteropneusta, and Chordata, the body-cavity is formed in the fashion noted in the case of Sagitta by two ingrowths from the archenteron, the bulk of the mesoderm arising from the differentiation of the epithelium of these diverticula, though sometimes partially also from amoeboid cells budded off into the gelatinous tissue between ectoderm and endoderm. In the smaller group of Pseudocoela, including the Mollusca, Polyzoa, Botifera, and Platyhelminthes, the mesoderm originates in the second of these ways, and the body-cavity is formed by a split in this “mesenchyme.” This theory has been criticized by Balfour ( Embr., ii. 13), and more recently ably defended by Haeckel and others; further detailed studies are at least needed before it can claim full confirmation. (15)
4. Unity of Tissues and Homology of Layers. —Since it is impossible that all the cells resulting from segmentation can continue to occupy the same position, a certain lamination or layer formation with consequent division of labour and structural modification invariably occurs, finding its first expression in the two-layered gastrula stage to which we have referred. Just as in the embryo of the higher plants there arise an outer dermatogen, an inner plerome, and a middle layer or periblem, so in the animal embryo there constantly occur three germ layers, the ectoderm, the endoderm, and the intermediate layer formed from them—the mesoderm. As early as 1768 Wolff seems to have had a definite presentiment of the formation of the different systems from distinct germ layers; in 1817 Pander distinguished in the embryo chick the outer or “serous” from the inner or “mucous” layer, and Yon Baer in 1828 yet more definitely distinguished them as animal and vegetative respectively. A great step towards the recognition of the full significance of these germ layers was due to the brilliant insight of Huxley (1859) in comparing them to the ectoderm and endoderm which he had demonstrated in Ccelentera. The researches of Remak, Rathke, Kowalevsky, and others led to their general recognition throughout the Metazoa. Modern progress is marked by the demonstration of the general homology of all the three germ layers and consequently of the systems of organs which arise from them. While much uncertainty still obtains as to the origin of the mesoderm, and while the different layers undoubtedly exhibit in some cases the possibility of differentiation into tissues different from those to which they usually give origin, yet the general homology of the layers is now indisputable. The ectoderm, which is indeed the primitive sensory and protective organ, forms the epidermis, sense organs, and nervous system of the adult; the nutritive endoderm lines the enteric cavity, &c.; and from the mesoderm are derived the muscular, vascular, and lymphatic systems and the greater part of the connective tissue, as also the excretory and generally also the generative system. Not only then is the occurrence of these three layers constant, but, with few exceptions, so is the differentiation of each into definite systems of organs. This correspondence, which is of high—in most cases of final—importance in determining homologies, may be termed homodermic. See Morphology. (15)
5. Unity of Anatomical Structure .—The study of development leads, however, to the recognition of a more detailed and specific unity than the general one of homodermy. For, just as the development of the flower reveals the original resemblance of organs which become subsequently widely differentiated, so organs which in their adult modification seem hardly comparable are seen to be moulded from one pattern when compared in their embryonic simplicity. The same holds good obviously of the comparison of organs in different animals; organogeny is the necessary foundation of comparative anatomy. Thus of late years it has been demonstrated that the nervous system and sense organs, throughout at any rate the vast majority of Metazoan forms, not only constantly arise from the ectoderm—that is to say, are homodermic—but their development exhibits correspondences even in detail. Recent studies of the development of the Vertebrate skeleton, and more especially of the skull, afford vivid instances of that fundamental unity of structure of which, in another department, the final unravelment of the complex problem of the structure of the urinogenital organs of Vertebrates through the researches of Balfour and Semper is one of the most recent and admirable instances. (11)
6. Unity of Type .—It has been the general history of classification both of plants and animals that arrangements based on superficial resemblance were superseded by those founded on internal organization, while the latter have been in turn either corroborated or amended by the relationships revealed by the study of development, as from the preceding generalizations must indeed be obvious. Just as the separation of the Monocotyledons and Dicotyledons into two great alliances was anatomically recognized by Ray, and em-bryologically corroborated by De Jussieu, so the developmental studies of Von Baer led him independently to the establishment of the four great types which had been previously distinguished by the anatomical labours of Cuvier. And, apart from the recognition of the morphological unities of the greater groups, it is familiarly known how in the case of problematical forms, such, for instance, as Cirripedes or Ascidians, where the adult organization is obscured by degeneration, the detection of the true relationship is due to embryology. In proportion, too, as the recapitulation during development of adult structure becomes shortened and effaced, so does the determination of their real affinities become difficult; hence the relatively slow progress of the botanist towards a knowledge of the deeper affinities of his “natural orders.” (11)
As the preceding outline of embryological detail is being rapidly filled up, and the resultant concrete generalizations are being more and more clearly defined, and also united with those of comparative anatomy and palaeontology, the embryologist is rapidly approximating that tolerably adequate knowledge of the morphological relations of plants and animals alike which finds its graphic expression in the “family tree” of the Organisata. While, for instance, the discrepant speculations of Semper, Hubrecht, Sedgwick, and others, as to the ancestral form of the Chordata , afford illustration of how far we are from being able to construct a thoroughly definite genealogical tree, on the other hand no better evidence alike of the rapid stages by which our knowledge has advanced and of the utility of such a method of graphic notation can be obtained than by comparing the necessarily vague and hypothetical tree sketched by Haeckel only twenty years ago with any of its numerous and shortlived successors. (16)
3. The result of the study of development is not, however, merely to establish the existence of such concrete structural unities as those just indicated, but leads further to the recognition of certain abstract generalizations, expressive of the most comprehensive conclusions which can be drawn from the observed succession of developmental phenomena.
(a) Heredity .—It is an every-day observation that the offspring of any organism repeats the organization of the parent; and the very familiarity of the fact is apt to conceal the marvellousness of the process in which every egg cell develops, either directly or indirectly, into a form which not only resembles the parent in general and specific characters, but may even repeat those individual characteristics which arose by so-called spontaneous variation, or which were even impressed upon the parent by the direct influence of the external environment. The difficulty of analysing the factors which give rise to this result,—that is, of understanding how the history of the developing ovum is determined by its constitution—the uncertainty as to the degree in which acquired parental characteristics can be said to be transmitted, the absence in fact of any established causal explanation of the resemblance between offspring and parent, in specific and individual characters, does not of course affect the fact. Although, in analysing the popular generalization that “like begets like,” it may eventually be shown how much of that likeness may be due to the hammering of the same environmental forces which formerly played upon the parent, a mysterious transmission of properties has still to be accounted for, and interpreted in terms of the physiological and morphological, the chemical and physical, composition and properties of the germinal matter of parent and offspring. To explain this mystery, various “theories of heredity” have been from time to time propounded; from the present purely morphological point of view it suffices here, however, to note the fact of heredity, leaving the discussion of its rationale to its more natural place at the outset of the article Variation and Selection. (6) (b) Von Baer’s Law .—In comparing the degree of organization attained by different forms, we are accustomed to distinguish the general morphological ground-plan constant throughout the group from the detailed histological differentiation or elaboration of the various organs. Great histological simplicity may co-exist with a high general morphological plan, and vice versa. A survey of the animal and vegetable kingdom reveals a branching and ascending series of increasingly complex ground-plans, while, in any of the groups determined by these, similar branching series more and more highly differentiated in detail are to be distinguished. Von Baer (1828) was the first to discern the embryological aspect of this law of progress, and to show that, in the development alike of the organism and of its component parts, there was a progress from the simple to the complex, from the general to the special. Thus, in the development of one of the higher Mammalia , those characters which are common to the lowest Vertebrates are at first alone distinguishable. Thence the embryo passes through stages resembling those of successively higher forms, till the general Matnmalian type is reached, this again passing through higher and less general stages till the specific characters finally make their appearance; and this progressive differentiation from general to special holds equally of the histological differentiation of the organs. Von Baer guarded against the error involved in many popular versions of his generalization, by maintaining that the developing embryo resembled, not the adults, but merely the embryos of lower forms; and, although he narrowed his proposition to the limits of the great groups, denying, for example, any resemblance between Vertebrate embryos and those of any Invertebrate type, this must be admitted a thoroughly legitimate reserve when we consider the actual state and practical possibilities of embryological research at the time. The real value and import of Von Baer’s law, however, could not be appreciated until the parallel between this developmental progress and the advance of anatomical type which had been exposed by Cuvier received its rational explanation at the hands of Darwin. (17)
(c) Haeckel’s “Biogenetic Law .”—While Von Baer was the first to appreciate the value of embryology in its relation to classification and comparative anatomy, we are indebted to Haeckel for the detailed application of the Darwinian theory to the phenomena of embryology and the consequent restatement of Von Baer’s law in its developed form. In his Generelle Morphologie (18G6) he formulates the “fundamental law of development״ ( biogenetisches Grund-gesetz). Introducing the term “ontogeny” to denote the development of the individual organism, and “phylogeny” to express the historic evolution of the “phylon” or tribe, he affirms that “ontogeny is an epitome of phylogeny,” or, more explicitly, “the organism recapitulates in the short and rapid course of its individual development the most important of those form-modifications undergone by the successive ancestors of the species, in the course of their long and slow historic evolution, and the causal relation of the two histories is to be explained in terms of heredity and adaptation. When these are thoroughly analysed, it will be possible to say that the phylogeny is the mechanical cause of the ontogeny.”
Much as Von Baer had distinguished the general morphological ground-plan from the more detailed differentiations of the organs, Haeckel analysed ontogeny into (a) the “palingenotic” process, in which the truly ancestral characters conserved by heredity are reproduced in development, and (b) the “kenogenetic” process, or modified evolution, to which are due those non-primitive characters which have resulted in consequence of a secondary adaptation of the embryo to the peculiar conditions of its own environment. The true recapitulation is constantly liable to be disguised, not only by the frequent occurrence of that abbreviated and more direct ontogeny which the need for ־ economy tends alike to originate and to conserve, but by the action of these kenogenetic processes. Hence the corollary that “the ontogenetic recapitulation of the phylogeny is the more perfect the more the palingenetic process is conserved by heredity, and the more imperfect in proportion as the later modified evolution (kenogenesis) is introduced by adaptation.”
While this distinction between adaptive characters and underlying morphological type is not only legitimate but indispensable, it must not, however, be forgotten that the difference between these is nowhere absolute,—the deepest morphological characters being but the most ancient results of adaptation (cf. Morphology, vol. xvi. p. 845). Yet it is only by the careful application of this principle that the embryologist can unravel the perplexing entanglement of primitive and adaptive characters presented by so many larval forms, or solve the scarcely less difficult problems of organogeny. In this regard Balfour’s dissertation on the origin and affinities of larval forms is especially valuable, while a vivid illustration of the employment of the biogenetic law, in one of the most difficult departments of ontogeny, may be borrowed from Prof. Parker, lieviewing the development of the skull in the Chick, lie says—
“Whilst at work I seemed to myself to have been endeavouring to decipher a fralimpsest, and that not erased and written upon again just once, but live or six times over.
“Having erased, as it were, the characters of the culminating type—those of the gaudy Indian Bird—I seemed to be amongst the sombre Grouse, and then, towards incubation, the characters of the Sand-Grouse and Hemipod stood out before me. Rubbing these away, in my downward walk, the form of the Tiuarnou looked me in the face; then the aberrant Ostrich seemed to be described in large archaic characters; a little while and these faded into what could just be read off as pertaining to the Sea Turtle; whilst, underlying the whole, the Fish in its simplest Myxinoid form could be traced in morphological hieroglyphics.” (17)
(d) Spencer on Development.— The most generalized treatment of embryology is that of Spencer, who, after carefully distinguishing mere growth in bulk from development of structure, points out that development takes place primarily around a central point, as in the lowest and chiefly unicellular organisms. Central development is either unicentral or multicentral, while, according as the insubordination to a single centre, implied in the latter case, is more or less thorough, the organism is of irregular form (e.g., many Algue), and so may readily even become discontinuous. From central we pass insensibly to axial development, and this may be uniaxial or multiaxial. Here, too, development may be continuous or discontinuous, familiar instances of both being furnished by many animals and plants. The fundamental importance of these simple conceptions to the adequate treatment alike of the problems of individuality (cf. Morphology) and of the nature of the reproductive process is justly to be insisted upon, for the definition of reproduction as but a discontinuous growth and development finds here its origin and justification.
Spencer, moreover, expresses Yon Baer’s essential law in yet more general phrase :—“Development is a change from an incoherent indefinite homogeneity to a coherent definite heterogeneity.” The relation of ontogeny to phylogeny is not overlooked, and a yet farther parallel advance in differentiation of the organism from its environment is illustrated alike in structure and form, in chemical composition and specific gravity, in temperature and self-mobility. The deductive interpretation of these laws is also cautiously suggested. (17)
Bibliography. —Without any attempt to deal with the very copious literature of the subject, it is sufficient to name some of the more important general and special works, from which full details can in turn be obtained. From the time of Haller perhaps no eminent anatomist or physiologist has omitted a more or less general treatment of the subject, and such discussions as those to be found in the well-known works of Johannes Müller, Milne-Edwards, Owen, or Carpenter are still valuable, especially as embodying the past development of the subject. More recent discussions are to be found in the leading text-books, alike morphological (Huxley, Gegenbauer, Claus) and physiological (Hermann, Foster, Landois, &c.). The embryological movement can be followed by the aid of the article Embryology, and the valuable systematic treatise of Balfour, while the most generalized treatment of the subject must at first be sought in the works of Spencer and Haeckel. For almost all purposes, how r ever, the recent careful monograph of Hensen (Physiologie d. Zeugung, forming the second part of vol. vi. of Hermann’s Handbuch d. Physiologic, Leipsic, 1881), although, of course, by no means completely satisfactory, will be found not merely serviceable but indispensable to the student. The various Jahresberichte must be also, of course, constantly appealed to, especially for progress in detail. The present state of the theory of reproduction is discussed in Hensen, op. cit.
(1) cf. Hensen, op. cit., general manuals, and gynecological works, e.g., Williams, “On the Structure of the Mucous Membrane of the Uterus," Obstetr. Journ., 1875; Barnes, System of Midwifery, 1885; Lusk, Sei. and Art. of Midwifery, 1882. (2) Huxley, .׳tnai. of Invert. Animals, 1879; Gegenbaur, Comp. Anat., London, 1882; Wiederslieim, Le/irb. d. Comp. Anal. d. Wirbelthiere, Jena, 1883; Semper, D. Vrinogenital Syst. d. Plagiostomen, in his Arbeiten. vol. ii.; Balfour, Comp. Embryology, 1882. (3) Hensen, op. cit.; Foster’s and
Landois’s manuals of physiology. (4) Werth, “Physiologie d. Geburt," in chap, xiii. of Hensen, op. cit.; Spiegelberg, Lehrb. d. Geburtshülfe, Lahr, 1878. (5)
Milne-Edwards, Logons s. la Physiologie, and later manuals. (6) Hensen, op. cit. (7) Gamgee, Physiol. Chem. of Anim. Body, 1880, and Hensen, op. cit. (8) V. La Valette, “Ueb. d. Genese d. Samenkörper," Archiv f. Mikr. Anat., xv.; Blom-fiel·!, Quart. Journ. Micro. Sei., 1880; Renson, Arch. d. Biol., 1882; Swaen and Masquelin, Arch. d. Biol., 1883; Geddcs and Arthur Thomson, “On the History and Theory of Spermatogenesis,” Proc. Boy. Soc. Edit!., 1885-8G. (9) See Embryology, and article “Ovum” (by Allen Thomson), in Todd’s Cyclop, of Anat. and Physiol. (10) Gamgee, op. cit.; Hensen, op. cit. (11) Balfour, op. cit., and manuals. (12) Balfour, op. cit.; Flemming, Zellsubstanz, &c., Leipsic, 1882; Strasburger, Zellbildung, &e., Leipsic, 1882, and Neue Untersuch., <tc., 1884; Carnoy, Biologie Cellulaire, Louvain, 1885; Brass, Beitr. z. Zellphysiologie, Leipsic, 1885; Hertwig, Jena. Zeitschr., xviii., Heft 2,1885. (13) Balfour, op. cit.; V. Beneden, Arch. d. Bio!., 1883 (also summaries in Q. J. M. S., and Biol. Central-Blatt for 1885, by Cunningham and Flemming respectively); Sabatier, Contrib. ä V etude d. globules polaires, Montpellier, 1884. (14) Woiks above cited amt Hertwig, Jena. Zeitschr., 1885. (15) Balfour, op. cit.; Haeckel, “Ursprung u. Ent-wick. d. thier. Gewebe," Jena.Zeitschr., xviii., 1885. (16) Haeckel, Gen. Morphol., 1866; Ilerdman, Phylogeny of A nimal Kingdom, Liverpool, 1885. (17) Spencer, Princ. of Biol., 1880; Haeckel, Gen. Morphol., I860; Parker, Morphology of the . Skull, 1877. (P. GE.)
IL Reproduction of Plants.
The various modes by which plants are reproduced may be conveniently classified in two groups, namely, vegetative multiplication and true reproduction, the distinction between them being this, that, whereas in the former the production of the new individual may be effected by organs of the most various kinds, in the latter it is always effected by means of a specialized reproductive cell. This distinction will become apparent in the following discussion.
Vegetative Multiplication. —The simplest case of vegetative multiplication is afforded by unicellular plants. When the cell which constitutes the body of the plant has attained its limit of size, it gives rise to two, either by division or gemmation; the two cells then grow, and at the same time become separated from each other, so that eventually two new distinct individuals are produced, each of which precisely resembles the original organism. A good example of this is to be found in the gemmation of the Yeast plant. This mode of multiplication is simply the result of the ordinary processes of growth. All plant-cells thus grow and divide at some time of their life; but, whereas in a multicellular plant the products of division remain coherent, and add to the number of the cells of which the plant consists, in a unicellular plant they separate and constitute new individuals. In more highly organized plants vegetative multiplication may be effected by the separation of the different parts of the body from each other, each such part subsequently developing the missing members and thus constituting a new individual. This takes place spontaneously, and in a marked manner in Mosses. The main stem gradually dies away from behind forwards; the lateral branches thus become isolated, and constitute new individuals.
The remarkable regenerative capacity of plant-members is largely made use of for the artificial propagation of plants. A branch removed from a parent-plant will, under appropriate conditions, develop roots, and so constitute a new plant; this is the theory of propagation by “cuttings.” A portion of a root will similarly develop one or more shoots, and thus give rise to a new plant. An isolated leaf will, in many cases, produce a shoot and a root, in a word, a new plant; it is in this way that Begonias, for instance, are propagated. The production of new plants from the leaves occurs also in nature, as, for instance, in the so-called “viviparous” plants, of which Bryophyllum calycinum (Crassulaceae) and many Ferns (JV eplirodium \Lastraea\ Filix-mas, Asplénium \Athyrium ] Filix-foemina, and other species of Asplénium) are examples. An interesting case of the vegetative development of new individuals from other plant-organs is afforded by Strasburger’s observations on Coelebogyne ilicifolia, Funkia¡ Nothoscordum fragrans, and Citrus; he found, namely, that in these plants, an adventitious formation of r embryos takes place by budding from the tissue of the nucellus bounding the embryo-sac. But it is in the Mosses, of all plants, that the capacity for vegetative propagation is most widely diffused. Any part of a moss, whether it be the stem, the leaves, the rliizoids, or the sporogonium, is capable, under appropriate conditions, of giving rise to filamentous protonema on wdiich new moss-plants are then developed as lateral buds.
In a large number of plants we find that provision is made for vegetative multiplication by the development of f more or less highly specialized organs. In Lichens, for instance, there are the soredia, wdiich are minute buds of the thallus containing both algal and fungal elements; these are set free on the surface in large numbers, and each grows into a thallus. In the Characeae there are the bulbils or “starch-stars ״ of Chara stelligera, which are underground nodes, and the branches with naked base and the pro-embryonic branches found by Pringsheim on old nodes of Chara fragilis. In the Mosses small tuberous bulbils frequently occur on the rhizoids, and in many instances ( Bryum annotinum, Aulacomnion androgynum, Tetraphis pellucida , <kc.) stalked fusiform or lenticular multicellular bodies containing· chlorophyll, termed gemmae, are produced on the shoots, either in the axils of the leaves or in special receptacles at the summit of the stem. Gemmae of this kind are produced in vast numbers in Marchantia and Lunularia among the Liverworts. Similar gemmae are also produced by the prothallia of Ferns. In some Ferns (e.g., Nephrolepis tuberosa and N. undulata) the buds borne on the leaves or in their axils become swollen and filled with nutritive materials, constituting bulbils which fall off and give rise to new plants. This conversion of buds into bulbils which subserve vegetative multiplication occurs also occasionally among Phanerogams, as, for instance, in Lilium bulbiferum. But many other adaptations of the same kind occur among Phanerogams, notably among annuals. Bulbous plants, for instance, produce each year at least one bulb or corm from which a new plant is produced in the succeeding year. In other cases, as in the case of the Potato, tubers are developed from subterranean shoots, each of which in the following year gives rise to a new individual. In other cases, again, as in Dahlia, Thladiantha dubia, &c., tuberous swellings are found on the roots, from each of which a new individual may spring.
True Reproduction. —It was mentioned above that the true reproduction of plants is effected by specialized cells; these cells may be generally designated spores. The structure of a spore is essentially this: it consists of a nucleated mass of protoplasm, enclosing starch or oil as reserve nutritive material, usually enclosed by a cell-wall. In those cases in which the spore is capable of germinating immediately on its development the cell-wall is a single delicate membrane consisting of cellulose; but in those cases in which the spore may or must pass through a period of quiescence before germination the wall becomes thickened and may consist of two layers, an inner, the endospore, wdiich is delicate and consists of cellulose, and an outer, the exospore , which is thick and rigid, frequently darkly coloured and beset externally with spines or bosses, and which consists of cutin. In some few־׳ cases among the Fungi multicellular spores are produced; these approximate somewhat to the gemmae mentioned above as highly specialized organs for vegetative multiplication. In some cases, particularly among the Algae and also in some Fungi (Peronosporese, Saprolegnieoe, Myxomycetes, Cliytridiaceoe), spores are produced wdiich are for a time destitute of any cell-w T all, and are further peculiar in that they are motile, and are therefore termed zoospores; they move, sometimes in an amoeboid manner by the protrusion of pseudopodia, but more frequently they are provided with one, two, or many, delicate vibratile protoplasmic filaments, termed cilia, by the lashing of wdiich the spore is propelled through the water. The zoospore eventually comes to rest, withdraws its cilia, surrounds itself wdth a cell-wall, and then germinates.
Spores are developed in various w־ays, and a prefix is in many cases added to the w r ord “spore” to indicate the nature of the process of development. Leaving details for subsequent discussion, we will now confine our attention to the main fact that all spores are developed in one or other of two ways, either asexually or sexually. In the former case a single reproductive organ gives rise to cells wdiich are capable, each by itself, of developing into a new r organism; such an organ is an asexual reproductive organ,
and such cells are asexually produced spores. In the latter case the reproductive organs are such that they do not singly give rise to cells capable, each by itself, of developing into a new organism. These are sexual reproductive organs. In some instances the sexual organ does not give rise to reproductive cells at all until it has received into itself more or less of the protoplasmic contents of another different, at least physiologically, sexual organ; and the cells which it then produces are capable, each by itself, of developing into a new organism. In others the sexual organ produces reproductive cells without any such previous fusion of protoplasm, but the cells thus produced are incapable, each by itself, of developing into a new organism. Such cells are sexual reproductive cells. It is only by the fusion of two such cells, physiologically different, that a reproductive cell is formed which is capable of developing into a new organism. The fusion either of the protoplasmic contents of two different sexual organs or of two different sexual cells constitutes the sexual process. It may take place, according to circumstances, either within the organs or, in those cases in which sexual cells are produced and are set free, externally to them. The resulting cells are sexually produced spores.
In some exceptional cases the normal production, sexual or asexual, of spores does not take place, but the new organism is developed vegetatively from the parent. When the sexual y production of spores is suppressed, the case is one of apogamy ; when the asexual production of spores is suppressed, the case is one r. of apospory. The following are instances of apogamy: in certain Ferns ( Ptcris cretica, Aspidium falcatum) the prothallium produces no sexual organs, but the fern-plant rises vegetatively as a bud upon it. Apospory lias been observed to occur in Mosses and in some Ferns ( Athyrium Filix-foemina, Polystichum angulare): in Mosses a new plant may be developed vegetatively from the tissue of the sporogonium; in Athyrium the sporangium, instead of producing spores, develops into a fern-prothallium. In the Characeae apospory appears to be the rule. Apogamy and apospory will be more fully discussed subsequently in connexion with the subject of alternation of generations.
Asexual Reproduction. —Reproduction by means of asexually produced spores is common to nearly all families of plants. It is wanting, among the Algae, in the Conjugatae, the Fucaceae, and the Characeae; among the Fungi, in a few Peronsporeae ( Pythiurn vexans, Arto-trogus), in Ancylistes Closterii, in Aplanes Braunii, among the Saprolegnieae; and, among the Ascomycetes, in Ere-mascus, Sordaria (Hypocopra), Ascobolus furfuraceus, Pyronema ( Peziza) conjluens, Gynmoascus, the Collemaceae, and most other Lichen-Fungi.
In the simplest case the spore is developed from a single cell of the plant, which surrounds itself with the characteristic thick wall. This occurs only in plants of low organization; Nostoc and Bacillus are examples of it.
In other cases the contents of the cell undergo division, each portion of the protoplasm constituting a spore. Examples of this are afforded, among unicellular plants, by Yeast and Protococcus, and in multicellular plants by the Confervaceae, the Ulvaceae, and some Florideae.
In this case each cell, the protoplasm of which divides to form spores, may be regarded as a rudimentary reproductive organ of the nature of a sporangium. In more highly organized plants special organs are differentiated for the production of spores. In the majority of cases the special organ is a sporangium, that is, a hollow capsule iu the interior of which the spores are developed. In the Thallophytes the sporangium is a single cell. In the Muscineai it is a multicellular capsule; in Riccia, in which the structure of the capsule is simple, the whole of the internal cells give rise by division to spores; in other Liverworts, and in the Mosses, in which the structure of the capsule becomes progressively more and more complex, a portion only of the internal cells give rise to spores. In the Ferns, Equisetacem, and Lycopodiacese the sporangium is multicellular, but simple in structure. This is true also in the llhizocarpaj and in the Ligulatee (Selaginella, Isoetes), but in these plants there is this peculiarity that there are sporangia of two kinds,—some, namely, which produce one, or a few, large spores, macrospores, and are hence termed macrosporangia, and others which give rise to a large number of small spores, microspores, and are hence termed microsporangia. The Phanerogams also bear two kinds of sporangia, which have received special names :— the macrosporangium, which produces only one mature spore, is termed the ovule ; the microsporangium, which produces a large number of microspores, is termed the pollen-sac. In some cases among the Fungi the spores are: not produced in the interior of a sporangium, but are 1 formed by abstraction. This occurs in some Mucorini, 1 such as Chaetocladium, in the Ustilagineae, the Entomoph-' thorem, the Peronosporem, the Ascomycetes, the Rusts (Uredine<e), and the Basidiomycetes.
These asexually produced reproductive cells are com monly spoken of simply as spores, but in many cases some addition has been made to the word, or an altogether different name is applied to them, in order to mark some peculiarity in their mode of origin, to indicate their order of development, or to assign them without periphrasis to a particular group of plants. Thus, as has been mentioned, zoospores are motile spores unprovided, for a time at least, with a cell-wall; stylospores are spores which are developed, not in sporangia, but by abstriction as mentioned above; tetraspores is the name given to the spores of the Floride® to denote the fact that four spores are produced by the division of the mother-cell. The uredospores of the Uredinese are those which are produced during the summer, whereas the teleutospores of these plants are those which are formed in the autumn, at the end of the period of growth. It was the custom, at one time, to speak of the spores of Fungi as conidia; and at the present time the macrospores and the microspores of Phanerogams are better known as embryo-sacs and pollen-grains l’espectively.
The organs which give rise to the asexually produced spores are usually not confined to a particular part of the plant in the Thallophytes. Instances of this do, however, occur among the Ascomycetous Fungi—namely, in the Pyrenomycetes. Here the production of the stylospores takes place in definite receptacles known as pycnidia. In the vascular plants (Pteridophyta, Phanerogams), the development of sporangia, speaking generally, is confined to the leaves. In many of the Pteridophyta the sporan-giferous leaves do not differ in appearance from the foliage-leaves; but in other cases they undergo considerable modification, as in the Equisetacem, Marsiliacese, some species of Lycopodium and Selaginella, and notably in the Phanerogams. In the Phanerogams the modification is so great that the sporangiferous leaves have received special names; those which bear the microsporangia (pollen-sacs) are termed the stamens, and those which bear the macrosporangia (ovules) are termed the carpels. When the sporangiferous leaves are thus modified they are usually aggregated together, and such an aggregate of sporangiferous leaves constitutes a flower. Sexual Reproduction. —In nearly all classes of plants above the Protophyta spores are formed by a sexual process; and in those in which no such process can be detected its absence is due, not, as in the Protophyta, to the fact that sexuality has not yet been developed, but to its gradual disappearance. The phenomena of sexual reproduction will be most intelligibly stated by tracing them in the different main divisions of the Vegetable Kingdom—the Algae, the Fungi, the Arcliegoniata, and the Phanerogams.
The protophytic Algae are reproduced by asexually developed 3- spores, but iu some forms an indication is already given of the differentiation of these spores into sexual reproductive cells which takes place in the higher forms of the group. In Protococcus, for instance, zoospores are produced, but the zoospores are not all precisely similar. In some cases the protoplasm of the cell divides only once or twice, the result being the formation of two or four relatively large zoospores, macrozoospores·, in other cases the protoplasm divides a greater number of times so that a considerable number of relatively small zoospores, microzoospores, are produced. Functionally these zoospores are alike; they all come to rest, and form new Protococci. Amongst the Confervoidae, which are more highly organized plants than the protophytic Algai, we find forms, of which Ulothrix may be taken as the type, which likewise produce macrozoospores and microzoospores in their ־ cells. The macrozoospores of Ulothrix simply come to rest and germinate; they are distinctly asexual spores. The microzoospores may also do this, but not unfrequently they coalesce in pairs; the product of fusion, the zygospore, as it is termed, then develops into a Ulothrix filament. This fusion of two similar reproductive cells— this conjugation, as it is termed—is one of the simplest forms of the sexual process; the zygospore is then a sexually produced spore, and the two cells which conjugate to form it are spoken of as gametes,—planogametes when they possess cilia, aplanogamctes when they do not.
Comparing Ulothrix with Protococcus, we see that in both the macrozoospores are asexual reproductive cells, whereas the microzoospores of Ulothrix exhibit an imperfect sexuality, inasmuch as they may germinate without previous conjugation. The piano-gametes of Ulothrix are, however, to be directly connected with the microzoospores of Protococcus; that is to say, the gametes are to be traced back to asexual spores. This is a point of fundamental importance.
Similarly, in Botrydium, one of the Siphoneae, there are two kinds of zoospores, some of which are asexual and others sexual; the visible difference between them is, in this case, not one of size, but the gametes have two cilia and the zoospores only one.
In the conjugation which takes place in the above-mentioned plants the gametes are quite similar in form and size, and take an equal part in the formation of the zygospore. The first indication of the differentiation of sexual gametes is afforded by Edo- f carpus siliculosus and Scytosiphon, belonging to the Phaeosporeae. The zoospores of these plants are produced in well-defined sporangia, some of which are multilocular and others unilocular. Inasmuch as only the zoospores developed in the multilocular sporangia have been observed to conjugate, that is, to be gametes, those developed in the unilocular sporangia are probably asexual. This being so, the multilocular sporangia are to be regarded, not as mere sporangia, but as sexual organs ( Gametangia) producing sexual reproductive cells. The process of conjugation of the gametes is, according to Berthold, as follows :—the gametes are at first quite similar in every respect; some of them, however, soon withdraw their cilia and come to rest, whereas others remain actively motile; one of the still motile gametes then coalesces with one which has come to rest to form a zygospore. The gametes in this case behave differently in the process of conjugation: the one is passive, the other active; the former is to be regarded as the female, and the latter as the male reproductive cell. But even in this case the gametes, if they fail to conjugate, can germinate independently.
In Cutleria the sexual differentiation of the reproductive cells is more marked; the male and female cells are developed in distinct sporangia, which may be termed respectively antliericlia and oogonia, two male cells being formed in each antheriilium, and one female cell in each oogonium; the female cell is considerably larger than the male, but they are both planogametes, that is, conjugating cells which swim by means of cilia; the female cell, however, soon withdraws its cilia and comes to rest, and then conjugation takes place, with the formation of a zygospore, much in the same way as in Ectocarpus described above.
The next stage in the sexual differentiation of the Algae is to be found in such forms as Yolvox, Yaucheria, (Edogonium, Fucus, and the Characeae. In these plants, as in Cutleria, the reproductive cells are entirely incapable of independent germination; they have lost altogether that characteristic property of spores; as in Cutleria also they are developed in two kinds of sporangia—antheridia and oogonia—and they are very different from each other in their form, size, and behaviour. In most cases the oogonium gives rise to a single relatively large cell, the oosphere, which is at no time provided with cilia, and is not set free from the oogonium; in some of the Fucaceae, there may, however, be two (Pelvetia), four (Ozotliallia [Ascophyllum]), or eight (Fucus) oospheres produced in each oogonium, and in all the Fucaceae the oospheres are set free from the oogonium. The antheridium gives rise to a large number of small ciliated cells, the antherozoids, one of which subsequently fuses with the oosphere. The coalescence of two such highly differentiated sexual cells is termed fertilization, to distinguish it from the conjugation of similar sexual cells, and the product of fertilization is termed an oospore, to distinguish it from the product of conjugation, the zygospore. In these plants the antherozoids still retain the essential characters of planogametes, whereas the oospheres have lost them.
But it is not in all Algae that sexual reproduction is effected by the fusion of well-defined sexual cells. In the Florideae the sexual reproductive organs are well differentiated. The male organ is an antheridium which produces antherozoids; but these antherozoids are peculiar in that they have no cilia, and are surrounded by a cell-wall; they are frequently, on this account, termed spermatia. The female organ is termed a procarpium or carpogonium; it may consist of one or many cells, but in all cases it consists of a projecting filament, the trichogyne, and a more expanded basal portion. It is peculiar in that no well-defined oosphere exists within it. Fertilization is effected by the antherozoid (spermatium) being passively brought into contact with the trichogyne; complete fusion then takes place, the contents of the spermatium passing into the trichogyne; the trichogyne now withers, and changes take place in the basal portion of the procarpium, one or more of the cells termed carpogenous cells divide, and by a process of budding give rise to a cluster of cells which are capable of germinating, and produce new plants; these are termed carpospores. At the same time the cluster of spores frequently becomes surrounded by an upgrowth of tissue; the mature fructification is termed a cystocarp. From the female organ of the Florideae there are, then, formed, in consequence of fertilization, a number of reproductive cells, the carpospores, each of which corresponds to the oospore of plants like Fucus, Vaueheria, &c. A more detailed comparison with Fucus makes this correspondence at once apparent. In Fucus the contents of the female organ (oogonium), which is a single cell, divide into eight oospheres, which are subsequently fertilized; in the Floridese there is, before fertilization, no differentiated oosphere, but the organ, as a whole, is fertilized by the spermatium, and it is in this case after, and not before, fertilization, as in the case oí Fucus, that a ]irocess of cell-formation takes place in the female organ; hence the reproductive cells formed by the fertilized female organ of the Floridese are at once fertile, and correspond to the fertilized oospheres (oospores) of Fucus.
The peculiarities of the sexual reproduction of some of the Floridese are of sufficient general interest to be mentioned here.
In the Coralline», according to Solms-Laubach, the procarpia are produced several together in a conceptacle; it is, however, only the central procarpia of the group which are capable of being fertilized, and the peripheral procarpia which produce carpospores. After the fertilization of the central procarpia, the carpogenous cells of the whole of the procarpia fuse together to form one large cell from the periphery of which the carpospores are produced by budding.
This physiological division of labour is more marked in Dud-resnaya, and a few other Florideae. In these plants some of the procarpia are destitute of a trichogyne, whereas others possess that organ. The spermatia fertilize those procarpia which possess a trichogyne, but these procarpia do not produce carpospores; but there grow out from them filaments which fertilize the procarpia destitute of a trichogyne, and these then give rise to carpospores.
The development of the carpospores in the Bangiaceae (Bangia, Porphyra) is peculiar. The carpogenous cell does not in this case, as in other Floridese, produce spores by budding, but its protoplasm divides into eight portions; these are set free as naked masses of protoplasm, which move about for a time in an amoeboid manner and then come to rest and surroimd themselves with a cell-wall.
In some of the higher Algse, namely, in the Sphacelarieae and in the Laminariese, families belonging to the Phaeosporeae, no sexual process has been observed as yet; but, as our knowledge of the life-history of these plants is imperfect, it cannot be definitely stated at present that they are entirely asexual.
In the somewhat aberrant group of the Conjugatae the sexual process is peculiar. Iu the Desmidiese and the Mesocarpeae it is effected in this way, that two adjacent cells, belonging usually to different filaments, throw out corresponding lateral protuberances which meet, and, the intervening walls being absorbed, form a canal placing the cavities of the two cells in direct communication; the protoplasm of each cell contracts, forming an aplanogamete, and travels into the canal, where the two masses meet and fuse. This is clearly a process of conjugation, similar to that of planogametes, and the product is likewise a single cell which is termed a zygospore. In the Zygnemeae, of which Spirogyra is a familiar example, the process is slightly different. Here the protoplasm of one of the two conjugating organs contracts first and passes over into the cavity of the other, there to fuse with its protoplasm.
Turning now to the Fungi, we find that in the simplest forms (Scliizomycetes, Saccharomycetes) there is no trace of sexual reproduction, whereas in the higher forms, with some exceptions, sexual reproductive organs are present, though they are in many cases functionless.
The lowest Fungi in which a sexual process has been observed are the Chytridiacese ( Polyphagus EugUnm), the Mucorini, and the Entomophthorere. In these, when it occurs, it takes the form of conjugation, with the production of zygospores. Conjugation is here effected in essentially the same manner as that described above for the conjugation among the Alg;c, by the fusion of two similar sexual organs. Conjugation of planogametes has been observed by Sorokin in Tetrachytrium and in Haplocystis, probably belonging to the Chytridiacea?, a fact of some interest as it is the only case of this form of the sexual process known in the Fungi.
In Protomyces and the Ustilaginere a process takes place which appears to be of a sexual nature, resembling the conjugation of the Mucorini. Certain reproductive cells of an elongated form, termed sporidia, are produced, which become connected by a transverse canal so that they then resemble the letter H. No zygospore is formed, but the conjugated sporidia are its equivalent. The j question of the sexual nature of this process is still under discussion, but it is made highly probable by the fact that, in all fully investigated cases, the sporidia are incapable of independent j germination.
In the allied groups of the Peronosporeae and Saprolegnie® two kinds of sexual organs are present, male and female, which correspond to the antlieridia and oogonia of the Algae. The female organ is here also termed an oogonium, and, like that of the Alga;, it may produce one oosphere (Peronosporeae) or many (most Saprolegniese). The male organ is also usually termed an anthe-ridium, but it is sometimes spoken of as a pollinodium on account of the mode in which it effects the fertilization of the oosphere. In one of the plants allied to these groups—namely, in Monoble- \ pharis spheerica, according to Cornu—the protoplasm of the antheridium becomes differentiated into motile antherozoids; this is the only case of the kind known among the Fungi. In the other members of these groups in which fertilization takes place the antheridium lies in contact with the oogonium, and produces at its apex a delicate tubular outgrowth, which bores its way through the wall of the oogonium and comes into contact witlvan oosphere; the tube then opens, and protoplasm from the antheridium passes through into the oosphere and fuses with it to form an oospore.
In the Ascomycetes sexual organs are very commonly present, t but it is only in a few cases that a sexual process has been observed actually to take place. The sexual oi’gans differ considerably in their form in the different genera. In some ( e.g., Gymnoascus, Eremascus, Eurotium, Penicillium) the sexual organs are similar; they are unicellular or multicellular hyphse, but in some the female organ, termed in the Ascomycetes the ascogonium or carpo-gonium, may be distinguished from the male organ, which is a pollinodial antheridium like that of the Peronosporese, in that it is wound into a close spiral. In others {e.g., Erysiphese, Ascobolus, Pyronema {Pezizd\ confluens) the sexual organs are readily distinguishable. In the Erysipheae and in Pyronema the ascogonium is a single relatively large ovoid cell; that of Pyronema produces a delicate tubular outgrowth, the trichogyne; the antheridium also is unicellular, but it is more slender. In Ascobolus the ascogonium consists of a row of five or six relatively large cells; the antheridium is a slender multicellular hypha. In all these cases the sexual organs are developed in such close proximity to each other that they come into contact. In other Ascomycetes belonging to the Discomycetous Lichens (Collema, Synechoblastus, Leptogium, Physma), and to the Pyrenomycetes (Polystigma), the antheridial filaments, termed sterigmata, are developed at a distance from the female organ in separate receptacles, spermogonia. In this case the formation of male cells is a necessity. Accordingly cells, termed spermatia, are produced from the sterigmata by abstraction, which resemble the male cells of the Florideae in that they are non-motile and have a cell-wall.
With regard to the process of fertilization, it must be premised that in no Ascomycete is the protoplasm of the ascogonium ever differentiated into an oosphere. When the sexual organs are ascogonia and pollinodial antheridia, fertilization takes place by the fusion of the undifferentiated protoplasmic contents of the two organs, a mode of fertilization which recalls the conjugation in the Mucorini, and in the Conjugatie among the Algxe. This has only been observed to take place in Eremascus and in Pyronema; in the latter the antheridium applies its apex to the trichogyne of the ascogonium, the intervening walls are absorbed, and the protoplasm of the two organs coalesces. Although this process of fertilization resembles the conjugation occurring in the Mucorini, the product is very different. The product of conjugation in the Mucorini is a single cell, the zygospore; the product of fertilization in the Ascomycetes is a “lumber of cells, termed ciscospores or carpospores. Thus in Eremascus the product of fertilization is a unicellular capsule, the ascus, in which eight ascospores are formed. In Pyronema the fertilized ascogonium enlarges and • gives rise to a number of outgrowths which produce asci; at the same time a number of h} r plue grow up from below around the developing asci, some of which produce delicate filaments, termed paraphyses, which lie amongst the asci, whereas others form an investing wall. The result is the formation of a fructification, termed an apothccium. Within each ascus eight spores are formed.
In those Ascomycetes which have spermatia fertilization is effected, as in the Floride* among the Algie, by the fusion of a spermatium with the trichogyne. The result is the same as in Pyronema: the fertilized ascogonium gives rise to hyphee which bear asci, and these, together with sterile liyphse, form a fructification.
It is of interest to note the similarity between the products of fertilization in these Ascomycetes and in the Florideie. In both cases the female organ produces no differentiated oosphere, and in both cases the product of its fertilization is a many-spored fructification. It was pointed out, in speaking of the Floridese, that each carpospore is the equivalent of a fertilized oosphere (oospore); this holds good also with regard to the carpospores (ascospores) of the Ascomycetes.
It may be that a similar sexual process takes place in the other forms mentioned above, viz., the Erysipheie, Penicillium, Sordaria, &c., but it has not been observed; in any case, the ascogonium in all these plants gives rise to asci and ascospores, and a more or less complex fructification is produced. But there is also some ground £ for believing that in some at least of these cases the sexual organs, c though morphologically differentiated, are functionless. For there 1 are clear indications of sexual degeneration in the Ascomycetes. j In some cases, for instance {e.g., Chsetomium, Melanospora), no ( antheridium can be distinguished, but the ascogonium eventually produces asci nevertheless. In others {e.g., Xylaria, as far as known at present) no male organ is produced, but there is an ascogonium which does not, however, give rise to asci; the asci, as well as the rest of the fructification, arise from the vegetative liyphiB. In others, again (e.g., Claviceps, Cordiceps, Pleospora), all trace of the sexual organs has disappeared, but a fructification containing ascospores is produced, as in Xylaria, from the vegetative hyphse. In others, finally, no ascospores are known, the only reproductive cells being the characteristic asexually produced stylospores.
In the remaining groups of Fungi, the Uredineae and the Basidio-mycetes, no sexual reproduction is known. In the Basidiomycetes no kind of sexual organ has been discovered. In the Uredinese spermatia are commonly produced, as in the Ascomycetes mentioned above, but no female organ is known; however, fructifications termed secidia are in some cases developed. These resemble somewhat those of the Ascomycetes, but differ in that here the spores ( secidiospores) are fonned by abstraction, and not in asci as in the Ascomycetes.
It may be suggested that the ascospores of the asexual Ascomycetes and the secidiospores of the Uredinese should not be included in an account of the sexual reproduction of the Fungi. It is true that these spores are asexually produced, but their evident homology with the sexually produced spores makes it inconvenient to treat of them apart. They differ from the sexually produced spores in that they are developed apogamously.
Under the name Archegoniata we may conveniently group together the Muscinese and the Vascular Cryptogams (Plerido-pliyta). The sexual organs, as also the process of fertilization, are essentially the same throughout. The female organ produces a single oosphere, and is termed an archcgonium ; it is essentially similar to the oogonium of the Tliallophytes, the only difference being that, whereas the archcgonium is multicellular, the oogonium is unicellular. The male organ, here also termed the antheridium, is likewise multicellular, and gives rise to a larger or smaller number of motile antherozoids. Fertilization is effected by the fusion of an antherozoid with the oosphere, which then clothes itself with a cell-wall and becomes an oospore.
In the Phanerogams the sexual organs are essentially of the nature of archegonia and of antheridia, but they are somewhat! modified and are called by other names. The female organ of the Gymnosperms, termed a corpusculum, closely resembles the archcgonium of the Archegoniata, and produces a single oosphere. In the Angiosperms, the female organ is much reduced, consisting only of three cells, one of which is the oosphere, the other two being the synergicla which assist in the process of fertilization; the organ is termed the egg-apparatus. The male organ in the Phanerogams is a unicellular filament termed the pollen-tube; its protoplasm does not undergo differentiation into antherozoids. The sexual organs of the Phanerogams recall those of the Perono-sporca; and the Saprolegnieae; in both cases the female organ produces an oosphere, and in neither does the protoplasm of the antheridium produce antherozoids. The process of fertilization will be described subsequently.
Physiology of Reproduction .—From the fact that in even the most highly organized plants an isolated portion of one member is capable of producing, not merely a member like itself, but other members also, so that a new individual is constituted, it is clear that the protoplasm of plants is imperfectly differentiated physiologically. Nevertheless all plants produce cells to which the work of reproduction is especially assigned. It is of interest to recall the fact that a suppression of spore-formation, either asexual or sexual, may occur, and vegetative multiplication be reverted to, as in aposporous and some apogamous plants.
It has been shown above that the reproductive cells of plants are of two kinds—those, namely, which are individually capable, and those which are individually incapable, of giving rise to a new organism; the former are the asexual, the latter the sexual reproductive cells. It has also been indicated that the latter are to be regarded as derivatives of the former, a point which may now be somewhat more fully established. It was pointed out, namely, that the gametes of Ulothrix will, if they fail to conjugate, germinate independently; the sexual differentiation of these gametes is clearly imperfect, and they differ but little from asexual zoospores. The same thing has been observed in Botrydium, and this is a specially interesting case inasmuch as it throws some light upon the conditions which determine sexual differentiation of the reproductive cells in these lowly organized plants. It has been ascertained that the nature of the cells produced from the resting-spore, in the manner described above, depends upon the age of the spore producing them: when the spore is young, the cells produced by it are sexual gametes; if they fail to conjugate they perish; when the spore is old, the cells produced by it are entirely asexual zoospores; they never conjugate, but each by itself gives rise to a new individual. The imperfect sexual differentiation of the gametes has also been observed in Ectocarpus; if they fail to conjugate they germinate independently. The occurrence of this in Ectocarpus is rather surprising when it is remembered that the gametes of this plant are to some extent sexually differentiated as male and female (see above).
From these cases in which the typically sexual reproductive cells still possess the properties of asexual spores we pass to others, like Acetabularia, in which they have entirely lost these properties. The planogametes of this plant are definitely sexual; but they are quite similar, as far as external appearance goes, to each other; there is no perceptible distinction between male and female cells. This is the case also in the Mesocarpeae and the Desmidieae among the Conjugate; here the non-ciliated conjugating masses of protoplasm ( aplanogametes) are externally similar ־ and take an equal part in the sexual process. In Cutleria lf the planogametes, and in the Zygnemese the aplanogametes, c give indications of further sexual differentiations; in Cutleria the female gamete is much larger than the male and comes sooner to rest; in the Zygnemese the one aplanoga-mete passes over into the cell producing the other aplano-gamete and fuses with it; the former is to be regarded as male, the latter as female. Finally, in the oosporous Algae, in the Muscinese, and in the Pteridophyta the two cells are quite distinct in form, size, and behaviour; the male cell (antherozoid) alone retains the character of a planogamete, the female (oosphere) is non-motile and is many times larger than the antherozoid. In this series the gradual differentiation of the highly differentiated sexual cells from asexual cells can be clearly traced.
If the sexual reproductive cells are to be traced back to asexual spores, then the organs which produce the sexual reproductive cells are also to be traced back to those which produce the asexual spores, namely, the sporangia; the most highly differentiated sexual organ—the antheridium, the oogonium, the archegonium, the carpogonium—is derived from the sporangium.
The question now arises as to the nature of the difference between sexual and asexual reproductive cells. It would appear that the former are in some way incomplete, that something is lacking to them which the latter possess, and that this lack is supplied in the sexual process. In many cases facts have been observed in connexion with the development of the sexual cells which indicate that they are thus incomplete. In Acetabularia the whole of the protoplasm of the gametangium is not used up in the formation of the gametes, and in the Peronosporeae only a portion of the protoplasm of the oogonium forms the oosphere; the remainder is simply the periplasm. In Vaucheria and other Algae a mass of protoplasm escapes from the oogonium when it opens. In other cases a process of cell-division has been observed to accompany the formation of the oosphere which recalls the production of the “polar bodies” in the developing eggs of animals. In the Archegoniata the central cell of the archegonium does not directly give rise to the egg, but a portion, the ventral canal-cell, is first cut off; this takes place also in the cor-pusc.ulum of most Gymnosperms. Similarly in the development of antherozoids, the whole of the protoplasm of the mother-cell is never used up in their formation.
In the germinating microspore of most of the Heterosporous Vascular Cryptogams and of the Phanerogams a process of cell-division takes place which Strasburger interprets as the formation of a polar body. The protoplasm of the microspore undergoes division so that two cells are formed, which may be distinguished as the vegetative and the generative, the former being much smaller than the latter in the Vascular Cryptogams and in the Gymnosperms, whereas in the Angiosperms the converse is the case; usually the separation of the two cells is permanent, but in most Angiosperms it is transitory, the only permanent indication of the cell-division being the presence of two nuclei in the pollen-grain; in some Gymnosperms two or three more vegetative cells may he cut off from the generative cell. The antheridium is in all cases formed from the generative cell. These vegetative cells Strasburger regards as of the nature of polar bodies. The nucleus of the generative cell undergoes division, to form in the Heterosporous | Vascular Cryptogams the nuclei of the mother-cells of the antherozoids, and in the Phanerogams the nuclei which take part in the sexual process, as will be more fully described below.
The assumption of the incompleteness of the sexual cells may be extended to those sexual reproductive organs which, like the procarpia of the Florideae, the pollinodial antheridia of the Peronosporeae, the ascogonium and pollinodial antheridia of the sexual Ascomycetes, do not give rise to differentiated sexual cells.
There is reason to believe that the sexual reproductive cells are spores which, by the loss of certain of their constituents, have undergone sexual differentiation, and that those sexual organs which directly take part in the sexual process without the intervention of reproductive cells are sporangia which have undergone sexual differentiation in the same way. This is finally proved by the fact that in cases in which the normal phenomena of sexual differentiation do not take place the reproductive cells can germinate without fertilization, and the female sexual organ can produce, without fertilization, cells capable of germination. These cases are examples of that form of apogamy which is known as parthenogenesis. Parthenogenesis in | plants producing differentiated sexual cells hasbeen observed in the Mucorini, the Entomoplithorese, and the Saproleg-nieae among the Fungi, and in Chara crinita among the Algae. In some Mucorini (occasionally in Absidia septata , A. capillata, Muco fusiger, Sporodinia, always in Mucor tenuis ) and in some Entomophthoreae, namely, the conjugating liyphae remained closed, and the protoplasmic contents of each surrounds itself with a cell-wall, the cells thus produced being quite similar to the normal zygospore; these cells are termed azygospores. In the Saprolegnieae and in Clmra crinita the oospheres behave like oospores and germinate in the same manner. The details of the development of these asexual sexual reproductive cells has been fully investigated by De Bary in the case of the oosphères of the Saprolegnieae. In this case that differentiation of the protoplasm of the oogonium into ooplasm and periplasm which has been described above as occurring in the closely allied and completely sexual Peronosporeae does not take place, but the whole is used in the formation of the oosphères. It is quite clear also that in the Saprolegnieae no sexual process takes place. In some species of Saprolegnia the antheridia are altogether absent, in others they are rudimentary, and even in those Saprolegnieae (some species of Saprolegnia, Achlya, Aphanomyces) in which the antheridia are well-developed they remained closed. No case is known of male parthenogenesis, that is, of the development of an individual from a male reproductive cell, among plants in which the sexual differentiation of the reproductive cells is well-marked, but there are instances of the kind in more lowly organized plants. Thus in the Mucorini, mentioned above, one of the azygo-spores produced may be regarded as belonging to a male organ; again, in Ectocarpus the male planogamete germinates independently as well as the female. Among plants the sexual organs of which do not produce specialized reproductive cells instances of parthenogenesis also occur. Thus the spores (ascospores) which are produced by the female organ (ascogonium) of those Ascomycetes, such as the Erysipheae, Pénicillium, Melanospora, Xylaria, in which the male organ is functionless or absent can only be regarded as being parthenogenetically produced.
Admitting, then, that sexual differ from asexual reproductive cells in the lack of something which the latter possess, and which was thrown off either from the former or from the organs which produce them, we may go on to inquire what this something may be. Our information on this point is very scanty, but Strasburger’s views throw some light upon it. He considers, namely,
• that the formative processes of the cell are regulated by the hyaline plasma of the nucleus—the nucleo-idioplasma, as he terms it; the richer the nucleus is in this substance the more capable is the cell of producing new cells. The asexual reproductive cells are then cells the nuclei of which are especially rich in this substance. He considers that the differences between asexual and sexual reproductive cells is quantitative and not qualitative—that the nuclei of the former are rich in nucleo-idioplasma, those of the latter poor, either originally or by the throwing off of part of their substance in the mode described above. In endeavouring to account for the further differentiation of sexual reproductive cells into male and female, it seems natural to suggest that the respective nuclei have undergone a qualitative differentiation, and that in the sexual process the qualitative, as w׳ell as the quantitative, incompleteness of each is made good. Strasburger is, however, strongly of opinion that this is not so, but that the difference is purely quantitative. But it must be pointed out that, according to this view, there is no reason why a sexual process should not take place, either between two male cells or organs, or between two female cells or organs, a possibility which is never realized, nor is it possible to account for the fact to be subsequently discussed that in many cases a sexual process cannot take place between sexual cells or organs of a closely-allied origin.
The nature of the sexual process will be better understood by a detailed description of it in some particular case. The following is a brief account of the results of Strasburger’s observations on the process of fertilization in Angiosperms.
At the period of the dehiscence of the anther, the protoplasm of the pollen-grain undergoes division into two cells—a smaller, the >- generative cell, and a larger, the vegetative cell. At the time of pollination, when the pollen-grain is mature, it is usually the case that the only persisting indication of the previous cell-division is the presence of two nuclei in the protoplasm. In many cases the two nuclei were found to differ qualitatively, inasmuch as the generative nucleus stained readily when treated with carmine, picro-carmine, methyl-green, &c., whereas the vegetative nucleus stained imperfectly or not at all. On the formation of the pollen-tube, the generative nucleus, and sometimes the vegetative nucleus also, is carried down into it, and the former then undergoes division into two; occasionally one of the new nuclei divides again. The pollen-tube grows down the style, enters the ovary, and is directed to the micropyle of an ovule. In the ovule the egg-apparatus lies at the micropylar end of the embryo-sac, and the delicate wall of the embryo-sac covering it undergoes absorption. In its further growth the pollen-tube comes into contact with the synergic!*; in some cases its growth now stops, and a portion of its protoplasm, with one or both of the generative nuclei, passes through the mucilaginous wall at the apex of the tube, without leaving any opening behind it, and travels to the oosphere; in other cases the pollen-tube grows between the synergidie to the oosphere, and at once pours a portion of its protoplasm with a generative nucleus into it. This is followed by the appearance of a second nucleus in the oosphere (the male pronucleus), which is the generative nucleus derived from the pollen-tube, which now travels towards the nucleus of the oosphere {female pronucleus) and fuses with it; wheu the two pronuclei have each a nucleolus these also eventually fuse. Fertilization is now complete.
The synergidie take no direct share in the process of fertilization, but become disorganized; their disorganization usually begins when the pollen-tube first comes into contact with them. They serve merely to direct the pollen-tube or its escaped contents to the oosphere and to provide nourishment for them.
The fusion of the male and female pronuclei has been observed also in the oosphere of the Gymnosperms, and in the conjugation of Spirogyra.
Since plants are so commonly able to reproduce vege-tatively, the question arises as to the necessity of the production, either sexually or asexually, of spores. The biological importance of these cells is very great. They are capable, namely, of retaining their vitality under external conditions, such as long drought, absence of food, extremes of heat and cold, which would prove fatal to the plant, and they therefore are essential to the maintenance of the species. Further they are of importance in the distribution of the species; they are light, readily transportable by wind or water, in some cases themselves actively motile. In this way they serve to prevent that close aggregation of individuals of the same species which would result from a continued vegetative multiplication, and would prove injurious to the species. In the case of Phanerogams, in which the macrospore is not set free from its sporangium, the same ends are obtained by the production of seeds.
The further question now arises as to the importance of the sexual process.
It appears that, if any given species, at least among the higher plants, is reproduced through a long series of generations in a non-sexual manner, the individuals tend to degenerate, and the original well-developed form can only be reattained by the formation of a sexually produced spore. This result is to some extent realized by the fusion of two sexual cells or organs belonging to the same individual—that is, by self-fertilization—but more completely when the fusion takes place between sexual cells or organs belonging to distinct individuals—that is, by cross-fertilization. In some plants, as in the Peronosporem and in those sexual Ascomycetes which have pollinodial antheridia, self-fertilization alone is possible; this is also the case in certain Phanerogams in which the arrangements are such (notably in cleistogamous flowers) that only pollen from its own anthers can reach the stigma of the flower. In most cases, however, the conditions under which the sexual process is carried on, such as the formation of free-swimming gametes and antherozoids, and of spermatia and pollen-grains which can be readily conveyed from place to place, are such as to favour the occurrence of cross-fertilization. In some there are special arrangements for the attainment of this end, the most general of which is dioecism, that is, the production of the male and female organs by distinct organisms. Thus in certain Fucaceae ( Fucus vesicidosus, F. nodosus ,
F. serratus, Ilimanthalia lorea) some individuals bear only antheridia, and others only oogonia; among the Muscineae the plants are frequently either male or female; in the Isosporous Vascular Cryptogams the prothallia are usually hermaphrodite; but exclusively male and female prothallia occur not unfrequently in the Filices and as a rule in the Equisetáceas. In the Heterosporous Vascular Cryptogams dioecism is brought about in a somewhat different manner: these plants have, as mentioned above, two kinds of spores, macrospores and microspores; the former on germination always give rise to a female (archegoniate) prothallium, the latter to a male (antheridial) prothallium; hence the male and female organs are necessarily borne on distinct organisms. In some Phanerogams even the two kinds of spores are produced by separate individuals, the flowers of the one producing pollen-grains (microspores) but no embryo-sacs, those of the other producing embryo-sacs (macrospores) but no pollen-grains. More special arrangements are to be found in the flowers of Phanerogams for ensuring cross-fertilization, and preventing self-fertilization, e.g ., the development of highly-coloured perianth-leaves and the secretion of nectar to attract insects, dichogamy, heterostylism, &c.; but it is impossible to do more than mention these here.
But besides these obvious structural arrangements for ensuring cross-fertilization and preventing self-fertilization there are in some cases imperceptible physiological conditions which even more certainly lead to the same results. It appears, namely, that in such cases no sexual process can take place between reproductive cells or organs of closely-allied origin. Thus, among the Algae, in Ecto-carpus and in Aceta bularía, conjugation only takes place between planogametes derived from distinct gametangia, and in Dasycladus it only takes place between piano-gametes derived from gametangia borne by different individuals. Again, in many Phanerogams, as Darwin has shown, the pollen of one flower is quite incapable of fertilizing the oospheres of its own ovules, and the pollen-grains from another flower borne by the same plant is but slightly, if at all, more potent. The pollen from the flower of another individual of the same species is potent, and this the more so the wider the difference between the individuals; the pollen from an individual of a different variety is more potent than that from an individual of the same variety.
The effect of the sexual process is not necessarily confined to the cells or organs which directly take part in it; not infrequently it makes itself felt in adjoining organs, stimulating them to active growth, giving rise to the formation of a fruit or fructification. Thus in the Mu-corini an outgrowth of filaments, forming an incomplete or complete (Mortierella) investment to the zygospore, takes place from the sexual organs after conjugation; in Coleochsete the oogonium becomes surrounded, after the fertilization of the oosphere, by an investment formed by outgrowths from the adjacent vegetative cells; a cellular investment is formed in the same way round the fertilized procarpium in most Florideae, and round the fertilized ascogonium in the Ascomycetes. The most familiar case of fruit-formation is that occurring in the Phanerogams; here in many instances the carpels, in some the floral leaves of the perianth, and in some the floral receptacle (torus) grow actively after the fertilization of the oospheres has taken place, giving rise to a mass of succulent parenchymatous tissue. In the Orchidese the development of the ovules does not take place at all until the flower has been pollinated.
Germination of the Spores and Alternation of Generations. —The spores of plants may either germinate immediately on their production, or they may undergo a· longer or shorter period of quiescence; those which are destined for immediate germination have, as described above, a thii cell-wall, whereas those which are capable of undergoing a period of quiescence have a thick cell-wall. In som! cases the spores are incapable of immediate germination notably sexually produced spores; for instance, amon¿ the Algae, immediate germination is only known to tak< place in the case of the zygospores of Botrydium anc Ectocarpus, and of the oospore of Fucus; among th< Fungi the zygospores of the Mucorini and the oospores ol the Peronosporese and Saprolegnieae pass through a perioc of quiescence.
The mode of germination is not always the same. Ir most cases the spore gives rise directly to a new organism, either by protruding one or more filamentous outgrowths or by the division of its protoplasm to form the tissue o: the embryo. In some cases the spore behaves like £ reproductive organ; from its protoplasm are formed £ larger or smaller number of cells, either motile or non motile, which are set free. In Acetabularia, and undei certain circumstances in Botrydium, the asexually pro duced spore behaves like a sexual reproductive orgai (gametangium), giving rise to a number of planogametes similarly the spore of Protomyces produces within itself £ number of conjugating sporidia. In some Peronosporea (always in Cystopus; occasionally, according to circum stances, in Pythium, Phytophthora, and Peronospora) th( asexually produced spore behaves like a sporangium, anc gives rise to a number of zoospores from each of which i new individual is developed. This happens occasionally also in sexually produced spores. Among the Fungi, th( formation of zoospores in the oospore occurs in various species of Peronosporeae and Saprolegnieae. Among the Algae, zoospores are formed in the zygospores of Pandorina and Ulothrix, and in the oospores of (Edogonium and Sphaeroplea. Cases of a similar kind are known also in the Phanerogams; thus in some Coniferae, and notably in the Gnetaceous Ephedra altissima, a process of cell-formation goes on in the oospore, resulting in the formation of a a larger or smaller number of cells from each of which an embryo plant is developed. These cases, in which, namely, the spore, whether sexually or asexually produced, gives rise to a number of cells, each of which is capable, by itself, of developing into a new individual, are instances of whal is known as polyembryony.
In some cases the cells formed in the sexually produced spore dc not each give rise to a new individual; this obtains in the Hydro-dictyese. In Hydrodictyon utricidatum the protoplasm of the zygospore gives rise to two or four large zoospores which eventually come to rest and remain quiescent for several months; these resting spores are termed, on account of their form, polyhedra. On germination, the protoplasm of the polyhedron gives rise to a number of small zoospores, the endospore protruding as a delicate vesicle, within which the zoospores are in active movement; the zoospores eventually come to rest, without escaping from the endospore, and arrange themselves so as to form the meshes of a small sac-like net, which is a young Hydrodictyon; the endospore is then disorganized, and the young net is set free a^ an independent ccenobium.
Occasionally it happens that a portion only of the spore gives rise to the embryo. This is the case in the sexually produced spores (oospores) of the Characese, and in those of the Coniferae; in Selaginella and in the Angiospermous Phanerogams one-half oi the oospore gives rise to a filamentous structure, the suspensor, the other half to the main body of the embryo.
It is very commonly the case that the spore, on germination, gives rise to an organism unlike that by which the spore was produced. In a Moss, for example, the asexually produced spores are developed by an organism, the sporogonium, consisting typically of a longer or shorter stalk, the seta, bearing a capsule (theca) which produces the spores. When one of these spores germinates, it does not give rise to another sporogonium, but to an inconspicuous, usually filamentous, structure, the protonema,
upon which are developed, as lateral buds, moss-plants consisting of stem and leaves and bearing the sexual reproductive organs. Similarly, when the oospore has been formed by fertilization in the archegonium, it does not give rise to the sexual moss-plant, but to the asexual sporogonium. There is thus in the life-history of a Moss a regular alternation of a sexual with an asexual generation; the former may be conveniently termed the oophore, the latter the sporophore·, the asexually produced spore always gives rise to the oophore (moss-plant), the sexually produced spore (oospore) of the moss-plant always gives rise to the sporophore (sporogonium).
This kind of life-history is not peculiar to the Muscinere, but it can be traced, more or less clearly, in all the vascular plants (Pteridophyta, Phanerogams). In the Isosporous Vascular plants, those, namely, which produced spores of one kind only (Filices, Equisetacem, Lycopo-diacese), the asexually produced spore gives rise, on germination, to a small inconspicuous organism consisting entirely of cellular tissue, termed the prothallium , on which the sexual reproductive organs, the antheridia and arcliegonia, are borne; the oospore produced by fertilization in the archegonium gives rise to the well-developed plant, consisting of stem, root, and leaves, which produces the sporangia and spores. The prothallium, derived from the asexually produced spore, is clearly the sexual generation or oophore; the fully-developed plant derived from the sexually produced spore is the asexual generation or sporophore. In the Heterosporous Vascular plants, those, namely, which produce spores of two kinds (Rliizocarpae, Ligulatse, Phanerogams), the spores likewise give rise to prothallia, though they may be rudimentary. The microspore gives rise to a prothallium which is reduced to a single antheridium, and which, with the exception of Salvinia among the Rhizocarps, and of the Phanerogams, does not project from the spore; in Salvinia and in the Phanerogams it projects from the spore in the form of a closed tube, which is known in the Phanerogams as the pollen-tube. Similarly the macrospore of these plants gives rise to a small cellular prothallium bearing one or more archegonia, which in the Rhizocarps extends beyond the limits of the spore, but does not become free from it; in the Ligulatae (Selaginella and Isoetes) the prothallium is only partially exposed by the rupture of the coats of the spore, and in the Phanerogams, w 7 here it is termed the endosperm, it remains permanently and completely enclosed within the spore (embryo-sac). In the Heterosporous Vascular plants, then, as in the Isosporous, the asexual generation or sporophore is that which is termed the plant, which is highly differentiated morphologically and histologically, and which produces the sporangia and the spores; the sexual generation or oophore is here represented by two prothallia, the one exclusively male, the other exclusively female, derived respectively from the microspores and the macrospores.
Such a life-history can be stated generally in the following manner:—twice in its course the individual consists of a single cell, the spore, which in the one case has been produced asexually, in the other sexually; the sexual generation (oophore) springs from the asexually produced spore, and gives rise to the sexually produced spore (oospore) from which the asexual generation (sporophore) is developed. It is of interest to note that in most cases the organism developed from the sexually produced spore is much more highly organized than that developed from the asexually produced spore.
A peculiarity of the macrospores of the Phanerogams is worthy of mention here, as it leads to the production of that structure, the seed, which is characteristic of Phanerogams; the production of a seed constitutes, in fact, the only real and constant distinction between Phanerogams and Cryptogams.
As a rule the asexually produced spores of plants become free from the sporangium in which they have been formed. In Phanerogams this is true only of the microspores (pollen-grains), the macrospores (embryo-sacs) remaining permanently enclosed in the sporangia (mucellus) producing them. This being the case, their germination, i.e., the formation of endosperm, must take place within them whilst enclosed in the sporangium; and, further, the formation of the female reproductive organs and the development of the embryo must take place under these circumstances also. The result is the production of a seed. In a typical seed three generations of the plant are represented; they are as follows :— sporangial tissue belonging to the parent sporophore = perisperm; tissue belonging to the oophore = endosperm; the new sporophore = embryo.
In the seed the development of the new asexual plant proceeds to a certain limit. When this is reached the development ceases, and it is only when the seed is placed under favourable conditions that the further development of the embryo, i.e., the germination of the seed, can take place.
It lias now been sliown that in the life-history of the. Muscineae and in plants above them in the vegetable 1 kingdom there is a regular alternation of generations, and ' the question now arises as to how far this is true of the! life-history of plants lower than the Mosses, that is, in the Thallophytes. It is clear that no such alternation can take place in the life-history of those which are known to reproduce only either sexually or asexually, nor in that of those individuals which produce spores both sexually and asexually, either simultaneously or at different times.
The following are instances of the above-mentioned possible cases :—
Thallophytes reproduced only by asexually produced spores :— Algae: Cyanophyceae or Phycochromaceae; Protococcaceae;
Spliacelariae and Laminarieae so far as known at present. Fungi: Schizomycetes; Saccharomycetes; Myxomycètes; some Chvtridieae; probably many Mucorini; a few Peronosporeae (probably Fhytophthora infestans and Pythium intermedium) ; some Ascomycetes and Ure-dineae; Basidiomycetes.
Thallophytes reproduced only by sexually produced spores :—
Algae: Conjugatae; Fucaceae; Sphaeropleae. The case of the Characeae will be subsequently discussed.
Fungi : a few Peronosporeae ( Pythium vexans, Artotrogus); Ancylistes Closterii; Aplanes Braunii among the Sapro-legnieae; some Ascomycetes, Eremascus, Sordaria (Hypocopra), Ascobolusfurfuraceus, Pyronema confluens, Gymnoascus, Collemaceae and other Lichen-Fungi; some Uredineae in which only aecidiospores are known.
Thallophytes in which the same form produces spores both asexually and sexually :—
Algae: Vaucheria, Hydrodictyon, Ulothrix, Oedogonium, some Florideae (e.g., Polysiphonia xariegata). Fungi : Mucorini; most Peronosporeae and Saprolegnieae; Monoblepharis; among the Ascomycetes, the Erysi-plieae, Eurotium, Pénicillium, Nectria; some Uredineae ( Uromyces appendiculatus, U. Bchenis, U. Scrophulariae, U. Cestri, Puccinia Berberidis).
In some of the Algae, as in the Volvocineae, in the life-history of which distinct sexual and asexual forms occur, no alternation of generations can be traced, since there is no certainty as to the nature of the form arising from any given spore; the form developed from the asexually produced spore is not, as in the typical life-history of the !Mess, necessarily sexual, nor is the individual produced from the sexually produced spore necessarily asexual. But in the life-history of some others an alternation of generations is traceable. Thus in the Siphonaceous Acetabularia the plant produces spores; these, as mentioned above, behave on germination as gametangia; the gametes conjugate to form a zygospore, and from the zygospore the asexual Acetabularia springs. Here there is a distinct and regular alternation of generations; the Acetabularia-plant is the asexual generation (sporophore), the gametangia alone representing the sexual generation (oophore). The life-history of Botrydium, another Siphonaceous Alga, is essentially the same as that of Acetabularia, but it is frequently less regular; thus the resting-spores, instead of producing gametes, may directly germinate to form a Botrydium-plant, a process which is clearly a case of apogamy; and, further, the Botrydium-plant does not necessarily produce resting-spores, but may produce zoospores by which it is directly reproduced. In Coleo-chsete, too, alternation of generations is indicated. The oospore produces by division a small individual which is always asexual, giving rise to zoospores which likewise produce asexual individuals; this asexual reproduction may continue through a number of generations until eventually a sexual individual is developed. In Coleo-chsete it is only in the case of the sexually produced spore that the nature of the resulting organism is known; it always gives rise to an asexual form, whereas the asexually produced spores give rise to an individual which may be either sexual or asexual. In the Characese the oospore always gives rise to an imperfectly developed form, the proembryo, which is to be regarded as the sporophore; however, it never produces spores, but gives rise to the sexual Chara-plant (oophore) vegetatively by budding.
The study of the life-history of the Fungi is attended with considerable difficulty, partly on account of the fact that in many cases the development of the sexual organs is dependent upon a combination of external circumstances which may but rarely present itself, and partly on account of there being frequently a great difference in habit between the sexual and asexual forms of the same plant, a difference which is sometimes accentuated, in parasitic Fungi, by the occurrence of the two forms on different plants as hosts ( keteroecism ). But there are Fungi in the life-history of which alternation of generations has been ascertained. Before entering upon an account of these, it must be stated that the term “sexually produced spore” will be applied not only to those the formation of which is known to be preceded by a sexual process, but also to those which are formed probably or actually without a sexual process—in a word, apogamously—but which may be considered, as pointed out above, to be homologous with those which are actually sexually produced.
In Mucor Mucedo and Phycomyces nitens among the Mucorini, for instance, the zygospore gives rise on germination to an imperfectly developed individual (promycelium) which is entirely asexual and produces spores; one of these spores, in turn, gives rise to an individual which produces spores asexually but may also bear sexual reproductive organs. Essentially the same life-history may be traced in certain Peronosporete ( Phytophthora omnivora, Pythium proliferum). In these cases the form developed from the sexually produced spore is always asexual, whereas that derived from the asexually produced spore may be sexual, but it always produces spores asexually; hence there is not a strict alternation of an asexual ancl a sexual generation. In others the alternation is complete. In the Ustilaginese, for instance, the asexually produced spore gives rise to an imperfectly developed mycelium ( promycelium), which is the sexual generation; it produces the sporidia, which conjugate in pairs, and from the product of conjugation springs the mycelium, which bears asexually produced spores.
Essentially the same life-history has been traced in some Ascomycetes and Uredineae. In Claviceps (Ascomy-cete) the sexually produced spore (ascospore) gives rise to an asexual form, long regarded as a distinct genus under the name of Sphacelia, from the spores of which the sexual form is reproduced. In Sclerotinia (Peziza) Fuckeliana (Ascomycete), a similar regular alternation of generations occasionally but not always occurs; the ascospore may give rise to an asexual form, long known as Botrytis cinerea, and when it does so the alternation of generations is complete; but it may give rise to another sexual generation, in which case no־ alternation takes place. In
Polystigma (Ascomycete), the ascospore gives rise to a promycelium which bears sporidia, and these sporidia give rise to the sexual form. In Endophyllum (Uredineae) the life-history is precisely the same as that of Polystigma: the promycelium is the sporophore generation, the mycelium developed from the sporidium the oophore. In other Uredineae the life-history is slightly modified in that asexually produced spores of at least two kinds make their appearance. The sexually produced spore (oecidiospore) gives rise to a mycelium which, in Gymno-sporangium and Hemipuccinia, bears asexually produced spores, teleutospores; in Puccinia Graminis the formation of teleutospores is preceded by that of somewhat different spores, the uredospores; in any case the teleutospore gives rise, on germination, to a second asexual generation, the promycelium, which bears sporidia; from these the sexual secidium-bearing generation is developed.
The rule that, in the alternation of generations, the alternate generations are developed from spores produced either sexually or asexually is not, however, without exceptions, for in some instances the one generation may spring vegetatively from the other without the intervention of a spore. This is brought about by the suppression either of the sexually produced spore or of the asexually produced spore; the former is an instance of apogamy, the latter of apospory.
Thus in the apogamous Ferns mentioned above, the asexual generation (sporophore) is developed as a bud upon the sexual generation (oophore); and in Botrydium the gametangium which is the representative of the sexual generation may, instead of producing gametes, produce zoospores, in which case the new asexual individual is not developed from a sexually produced zygospore, but from an asexually produced zoospore. Similarly, in the apo-sporous Ferns and Mosses and in the Characese the oophore is developed as a bud from the sporophore.
The alternation of generations may be also interfered with by a combination of apogamy and apospory. This is the case, namely, when one generation gives rise to its like by vegetative budding, sporophore to sporophore, oophore to oophore. For instance, when, as in the Phanerogams mentioned above (Coelebogyne, Funkia, Nothoscordum, Citrus), embryos are produced vegetatively from the tissue of the nucellus, that is, sporophore from sporophore, the typically intervening formation of spores, first, by the asexual method and, secondly, by the sexual method is suppressed. This is necessarily always the case among Phanerogams when one plant is produced vegetatively from anothei־. A striking instance of the same thing has been observed by Goebel in some species of Isoetes, in which an Isoetes plant was produced on the leaf in place of a sporangium. Similarly when oophore springs vegetatively from oophore, the typically intervening formation, first, of the sexually produced spore and, secondly, of the asexually produced spore, is suppressed. This occurs when, as mentioned above, a Moss-plant gives rise by budding or by means of gemma: to another Moss-plant, and when a Fern-prothallium gives rise to another by means of gemmte.
Bibliography. — General: Sachs, Text-look, 2d English edition, 1882; Goebel, Grundzüge der Systematik, 1882; Yines, “Alternation of Generations in Thallophytes,” Journal of Botany, 1879; Pringsheim, “Ueber den Generationswechsel bei den Thallophyten,” Jahrb. f. JViss. Bot., xi., 1877. Algae: Ealkenberg, in Schenk’s Handbuch der Botanik, ii., 1882; Yines, “The Pro-embryo of Chara,” Journal of Botany, 1878. Fungi: De Bary, Vergleichende Morphologie und Biologie der Pilze, 1884. Ferns: Cramer, “Ueber die geschlechtslose Vermehrung des Farn-prothaliums,” in Denkschriften der Schweitz. naturforsch. Gesellschaft, xxviii., 1880; Bower, “On Apospory in Ferns,” Jour. Linn. Soc., Botany, vol. xxi., 1885. Mosses: Pringsheim, Monatsbericht d. Akad. d. JViss. in Berlin, 1876; Stahl, Botanische Zeitung, 1876. Physiology : Strasburger, Ueber Befruchtung und Zelltheilung, 1878, and Neue Untersuchungen über den Befruchtungsvorgang bei den Phanero-gamen, 1884.
