The formative process through which the embryo passes is necessarily of very different degrees of complexity, according to the more simple or complex organization of the adult animal to which it belongs. But it presents throughout the whole range of animals certain general features of similarity dependent on the fundamental resemblance of the organized elements from which all animals derive their origin.

A minute mass of protoplasm constitutes not only the simplest, but also the invariable, form presented by the germinal part of the ovum or egg, and in all animals, except the Protozoa, in which the nature of the germ is still doubtful, it takes at first the form of an organized cell, or it is a definite spherical and nucleated mass of protoplasm. It is therefore a germ-cell.

In all ova the first stage of the formative process, following upon fecundation of the germ, consists in the multiplication of the egg or germ-cell by a process of the nature of fissiparous division, so that when this division has proceeded some length, it results in the production of a mass or congeries of organized cells descended from that which formed the primitive germ, and containing in combination the molecular elements of the materials contributed by the male and female parents to the formation of the fertilized germ. This is the mulberry stage, or morula, of Haeckel. In a more advanced stage among the higher animals, the cells of this mass assume more or less of a laminar arrangement, constituting the blastoderm or germinal membrane of

Pander and succeeding authors; and in the first and lowest forms of this structure two layers are distinguished, corresponding to the outer and inner cellular laminae of which the earliest form of the embryo consists in the higher, and the whole of the body in the lower, forms of animals. These layers are the ectoderm and endoderm of the embryologist and comparative anatomist (Huxley and Allman).

In the lowest animals little if any further differentiation of the germinal structures ensues; but in animals higher in the scale there arises a third or intermediate layer, the mesoderm, which takes an important part along with the other two layers in the formation of the animal organism. The cellular blastoderm, therefore, is already the embryo of the lowest animals; while in the higher that term could scarcely with propriety be applied to the product of development in the egg until some of the characteristic lineaments, however rudimentary, of the new animal are apparent.

But in the whole of this process of embryonic development, whether it be of the simplest or of the most complex kind, it is to be observed that it is solely by the multiplication and differentiation of cells which have descended more or less directly from the original germ-cell that the organizing process is effected. It follows from this that the processes of organic growth or embryonic development present a textural or histological uniformity to a remarkable degree throughout the whole zoological series. There is also a very striking similarity in the morphological phenomena of development within large groups of animals. Our knowledge, indeed, of the mode of formation of the young in all the varied forms of animal organization is still too limited to admit of our affirming that a uniform and progressive morphological type pervades the whole animal kingdom; but already many ascertained facts point strongly to such a conclusion, and the more our knowledge of the process of development in individual animals (ontogeny) advances, the greater resemblance do we recognize in the formative processes; so that it becomes more and more probable that the morphological development of any of the higher animals includes, or as it were repeats within certain limits, the various steps of the process which belong to the inferior grades of the animal kingdom. Hence we are led to the further conclusion that there is an essential correspondence between the individual development or ontogeny of the higher animals and the progressive advance of the organization in the whole animal series.

If, further, we adopt the Darwinian view of the evolution of animal life and organization by descent of one species of animals from others preceding it, we shall see that the embryological history of any animal is at the same time the history of its relation to other animals and of its phylogenetic development or gradual derivation as a species from more simple progenitors in the lapse of time. It is obvious, therefore, that we must look to the future progress of embryology as well as of palaeontology for a large portion of the facts upon which the confirmation of the modern theory of evolution will rest.

From what has been said it will be apparent that it would be impossible, within the limits of one article, to trace even in the briefest possible manner the phenomena of embryological development in all different animals. But special descriptions, so far as required, will find their appropriate places under the divisions of animals to which they respectively belong; and as there are some considerations relating to embryology which require to be stated besides the history of development, it has been deemed advisable to bring the more important facts of development of the embryo into connection with those relating to reproduction in general under the heading Génération, to which article, therefore the reader is referred.

In the present article, accordingly, we shall do no more than trace shortly the steps by which the modern science of Embryology has originated and has assumed the important position which it now occupies among the biological sciences.

In its scientific and systematic form embryology may be considered as having only taken birth within the present century, although the germ from which it sprung was already formed nearly half a century earlier. The ancients, it is true, as we see by the writings of Aristotle and Galen, pursued the subject with interest, and the indefatigable Grecian naturalist and philosopher had even made continued series of observations on the progressive stages of development in the incubated egg, and on the reproduction of various animals; but although, after the revival of learning, various anatomists and physiologists from time to time made contributions to the knowledge of the foetal structure in its larger organs, yet from the minuteness of the observations required for embryological research, it was not till the microscope came into use for the investigation of organic structure that any intimate knowledge was attained of the nature of organogenesis. It is not to be wondered at, therefore, that during a long period, in this as in other branches of physical inquiry, vague speculations took the place of direct observation and more solid information. This is apparent in most of the works treating of generation during the 16th and part of the 17th centuries.

Harvey was the first to give, in the middle of the latter century, a new life and direction to investigation of this subject, by his discovery of the connection between the cicatricula of the yolk and the rudiments of the chick, and by his faithful description of the successive stages of development as observed in the incubated egg, as well as of the progress of gestation in some Mammalia. He had also the merit of fixing the attention of physiologists upon general laws of development as deduced from actual observation of the phenomena, by the enunciation of two important propositions, viz.—(1) that all animals are produced out of ova, and (2) that the organs of the embryo arise by new formation, or epigenesis, and not by mere enlargement out of a pre-existing invisible condition [Exercitationes de Generatione Animalium, Amstelodami, 1651, and in English by G. Ent, 1853, London). Harvey’s observations, however, were aided only by the use of magnifying glasses (perspe-cillae), probably of no great power, and he saw nothing of the earliest appearances of the embryo in the first thirty-six hours, and believed the blood and the heart to be the parts first formed.

The influence of the work of Harvey, and of the successful application of the microscope to embryological investigation, was soon afterwards apparent in the admirable .researches of Malpighi of Bologna, as evinced by his communications to the Royal Society of London in 1672, “De ovo incubato,” and “De formatione pulli,” and more especially in his delineations of some of the earlier phenomena of development, in which, as in many other parts of minute anatomy, he partially or wholly anticipated discoveries, the full development of which has only been accomplished in the present century. Malpighi traced the origin of the embryo almost to its very commencement in the formation of the cerebro-spinal groove within the cicatricula, which he removed from the opaque mass of the yolk; and he only erred in supposing the embryonal rudiments to have pre-existed as such in the egg, in consequence, apparently, of his having employed for observation, in very warm weather, eggs which, though he believed them to be unincubated, had in reality undergone some of the earlier developmental changes.

The works of Walter Needham (1667), Regner de Graaf (1673), Swammerdam (1685), Vallisneri (1689)—following upon those of Harvey—all contain important contributions to the knowledge of our subject, as tending to show the similarity in the mode of production from ova in a variety of animals with that previously best known in birds. The observations more especially of De Graaf, Nicolas Steno, and J. van Horne gave much greater precision to the knowledge of the connection between the origin of the ovum of quadrupeds and the vesicles of the ovary now termed Graafian, which De Graaf showed always burst and discharged their contents on the occurrence of pregnancy.

These observations bring us to the period of Boerhaave and Albinus in the earlier part of the 18th century, and in the succeeding years to that of Haller, whose vast erudition and varied and accurate original observations threw light upon the entire process of reproduction in animals, and brought its history into a more systematic and intelligible form. A considerable part of the seventh and the whole of the eighth volumes of Haller’s great work, the Elementa Physiologice, published at successive times from 1757 to 1766, are occupied with the general view of the function of generation, while his special contributions to embryology are contained in his Deux Mémoires sur la formation du Coeur dans le Poulet, and Deux Mémoires sur la formation des Os, both published at Lausanne in 1758, and republished in an extended and altered form, together with his “Observations on the early condition of the Embryo in Quadrupeds,” made along with Kühlemann, in the Opera Minora (1762-68). Though originally educated as a believer in the doctrine of “preformation” by his teacher Boerhaave, Haller was soon led to abandon that view in favour of “epigenesis” or new formation, as may be seen in various parts of his works published before the middle of the century: see especially a long note explanatory of the grounds of his change of opinion in his edition of Boerhaave’s Praelectiones Academicae, vol. V. part 2, p. 497 (1744), and his Primae Lineae Physiologice (1747). But some years later, and after having been engaged in observing the phenomena of development in the incubated egg, he again changed his views, and during the remainder of his life was a keen opponent of the system of epigenesis and a defender and exponent of the theory of “evolution,” as it was then named—a theory very different from that now bearing the name, and which implied belief in the pre-existence of the organs of the embryo in the germ, according to the theory of encasement (emboîtement) or inclusion supported by Leibnitz and Bonnet. (See the interesting work of Bonnet, Considérations sur les Corps Organisés, Amsterdam, 1762, for an account of his own views and those of Haller.) The reader is also referred to the article Evolution in the present volume, for a further history of the change which has taken place in the use of the term in more recent times.

r It was reserved for Caspar Frederick Wolff (1733-1794), a German by birth, but naturalized afterwards in Russia, to bring forward observations which, though almost entirely neglected for a long time after their publication, and in some measure discredited under the influence of Haller’s authority, were sixty years later acknowledged to have established the theory of epigenesis upon the secure basis of ascertained facts, and to have laid the first foundation of the morphological science of embryology. Wolff’s work, entitled Theoria Generationis, first published as an inaugural Dissertation at Berlin in 1759, was republished with additions in German at Berlin in 1764, and again in Latin at Halle in 1774. Wolff also wrote a “Memoir on the Development of the Intestine” in Nov. Comment. Acad. Petropol., 1768 and 1769. But it was not till the latter work was translated into German by J. F. Meckel, and appeared in his Archiv for 1812, that Wolff’s peculiar merits as the founder of modern embryology came to be known or fully appreciated.

The special novelty of Wolff’s discoveries consisted mainly in this, that he showed that the germinal part of the bird's egg forms a layer of united granules or organized particles (cells of the modern histologist), presenting at first no semblance of the form or structure of the future embryo, but gradually converted by various morphological changes in the formative material, which are all capable of being traced by observation, into the several rudimentary organs and systems of the embyro. The earlier form of the embryo he delineated with accuracy; the actual mode of formation he traced in more than one organ, as for example in the alimentary canal, and he was the discoverer of several new and important embryological facts, as in the instance of the primordial kidneys, which have thus been named the Wolffian bodies. Wolff further showed that the growing parts of plants owe their origin to organized particles or cells, so that he was led to the great generalization that the processes of embryonic formation and of adult growth and nutrition are all of a like nature in both plants and animals. No advance, however, was made upon rhe basis of Wolff’s discoveries till the year 1817, when the researches of Pander on the development of the chick gave a fuller and more exact view of the phenomena less clearly indicated by Wolff, and laid down with greater precision a plan of the formation of parts in the embryo of birds, which may be regarded as the foundation of the views of all subsequent embryologists.

But although the minuter investigation of the nature and true theory of the process of embryonic development was thus held in abeyance for more than half a century, the interval was not unproductive of observations having an important bearing on the knowledge of the anatomy of the foetus and the function of reproduction. The great work of William Hunter on the human gravid uterus, containing unequalled pictorial illustrations of its subject from the pencil of Rymsdyk and other artists, was published in 17 75, and during a large part of the same period numerous communications to the Memoirs of the Royal Society testified to the activity and genius of his brother, John Hunter, in the investigation of various parts of comparative embryology. But it is mainly in his rich museum, and in the manuscripts and drawings which he left, and which have been in part described and published in the catalogue of his wonderful collection, that we obtain any adequate idea of the unexampled industry and wide scope of research of that great anatomist and physiologist.

As belonging to a somewhat later period, tut still before the time when the more strict investigation of embryologi cal phenomena was resumed by Pander, there fall to be noticed, as indicative of the rapid progress that was making, the experiments of Spallanzani, 1789; the researches of Autenrieth, 1797, and of Soemmering, 1799, on the human foetus; the observations of Senff on the formation of the skeleton, 1801; those of Oken and Kieser on the intestine and other organs, 1806; Oken’s remarkable work on the bones of the head, 1807 (with the views promulgated in which Goethe's name is also intimately connected); J. F. Meckel’s numerous and valuable contributions to embryology and comparative anatomy, extending over a long series of years; and Tiedemann’s classical work on the development of the brain, 1816.

Christian Pander’s observations were made at the instance and under the immediate supervision of Prof. Dollinger at Würzburg, and we learn from Von Baer’s autobiography that he, being an early friend of Pander’s, and knowing his qualifications for the task, had pointed him out to Döllinger as well fitted to carry out the investigation of development which that professor was desirous of having accomplished. Pander’s inaugural dissertation was entitled Historia meta-rnorphoseos quam ovum incubatum prioribus quinque diebus subit, Virceburgi, 1817; and it was also published in German under the title of Beitrage zur Entwickelungsgeschichle des Hühnchens im Eie, Würzburg, 1817. The beautiful plates illustrating the latter work were executed by the elder D’Alton, well known for his skill in scientific observation, delineation, and engraving.

Pander observed the blastoderm or germinal membrane of the fowl’s egg to acquire three layers of organized substance in the earlier period of incubation. These he named respectively the serous or outer, the vascular or middle, and the mucous or inner layers; and he traced with great skill and care the origin of the principal rudimentary organs and systems from different ones of these layers, pointing out shortly, but much more distinctly than Wolff had done, the actual nature of the changes occurring in the process of development.

Carl Ernest von Baer, the greatest of modem embryologists, was, as already remarked, the early friend of Pander, and, at the time when the latter was engaged in his researches at Würzburg, was associated with Dollinger as prosector, and engaged with him in the study of comparative anatomy. He witnessed, therefore, though he did not actually take part in, Pander’s researches; and the latter having afterwards abandoned the inquiry, Von Baer· took it up for himself in the year 1819, when he had obtained an appointment in the university of Königsberg, where he was the colleague of Burdach and Rathke, both of whom were able coadjutors in the investigation of the subject of his choice. (See V. Baer’s interesting autobiography, published on his retirement from St Petersburg to Dorpat in 1864.)

Vo∏ Baer’s observations were carried on at various times from 1819 to 1826 and 1827, when he published the first results in a description of the development of the chick in the first edition of Burdach’s Physiology.

It was at this time that Von Baer made the important discovery of the ovarian ovum of mammals and of man, totally unknown before his time, and was thus able to prove as matter of exact observation what had only been surmised previously, viz., the entire similarity in the mode of origin of these animals with others lower in the scale. (Epistola de Ovi Mammalium et Hominis Genesi, Lipsiae, 1827. See also the interesting commentary on or supplement to the Epistola in Heusinger’s Journals and the translation in Breschet’s Repertoire, Paris, 1829.)

In 1829 Von Baer published the first part of his great work, entitled Beobachtungen und Reflexionen uber die Entwickelungsgeschiehte der Thiere, the second part of which, still leaving the work incomplete,...