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...