EMBRYOLOGY 167 secured to embryology its permanent place among the biological sciences on the Continent ; vhile in this country attention was drawn to the subject by the memoirs of Allen Thomson (1831), Th. Wharton Jones (1835-38) and Martin Barry (1839-40). Among the more remarkable special discoveries which, belong to the period now referred to, a few may be mentioned, as, for example, that of the chorda dorsalis by Von Baer, a most important one, which may be regarded as the key to the whole of vertebral morphology ; the phenomenon of yolk segmentation, now known to be uni versal among animals, but which, was only first carefully observed in Batrachia by PreVost and Dumas (though previously casually noticed by Swammerdam), and was soon afterwards followed out by Busconi and Von Baer in fishes ; the discovery of the branchial clefts, plates, and vascular arches in the embryoes of the higher abranchiate animals by Eathke in 1825-27; the able investigation of the transformations of these arches by Eeichert in 1837; and the researches on the origin and development of the urinary and generative organs by Joannes Miiller in 1829-30. On entering the fifth decade of our century, the number of original contributions and systematic treatises becomes so great as to render the attempt to enumerate even a selection of the more important of them quite unsuitable to the limits of the present article. We must be satisfied, therefore, with a reference to one or two which seem to stand out with greater prominence than the rest as land marks in the progress of embryological discoveiy. Among these may first be mentioned the researches of Theodor F. W. von Bischoff, formerly of Giessen and now of Munich, on the development of the ovum in Mammalia, in which a series of the most laborious, minute, and accurate observa tions furnished a greatly novel and very full history of the formative process in several animals of that class. These researches are contained in four memoirs, treating separately of the development of the rabbit, the dog, the guinea-pig, and the roe-deer, and appeared in succession in the years 1842, 1845, 1852, and 1854. Next may be mentioned the great work of Coste, entitled Ifistoire gen. et particul. cht Developpement des Animaux, of which, however, only four fasciculi appeared between the years 1847 and 1859, leaving the work incomplete. In this work, in the large folio form, beau tiful representations are given of the author s valuable observations on human embryology, and on that of various mammals, birds, and fishes, and of the author s discovery in 1847 of the process of partial yolk segmentation in the germinal disc of the fowl s egg during its descent through the oviduct, and his observations on the same phenomenon in fishes and mammals. The development of reptiles received important elucida tion from the researches of Eathke, in his history of the development of serpents, published at Kb nigsberg in 1839, and in a similar work on the turtle in 1848, as well as in a later one on the crocodile in 1866, along with which may be associated the observations of H. J. Clark on the " Embryology of the Turtle," published in Agassiz s Contributions to Natural History, &c., 1857. The phenomena of yolk segmentation, to which reference has more than once been made, and to which later researches give more and more importance in connec tion with the fundamental phenomena of development, received great elucidation during this period, first from the observations of C. T. E. von Siebold and those of Bagge on the complete yolk segmentation of the egg in nematoid worms in 1841, and more fully by the observa tions of Kolliker in the same animals in 1843. The nature of partial segmentation of the yolk was first made known by Kolliker in his work on the development of the Cepha lopoda in 1844, and, as has already been mentioned, the phenomena were observed by Coste in the eggs of birds. The latter observations have since been confirmed by those of Oellacher, Gotte, and Kolliker. Further researches in a vast number of animals give every reason to believe that the phenomenon of segmentation is in some shape or other the invariable precursor of embryonic formation. A large body of facts having by this time been ascer tained with respect to the more obvious processes of de velopment, a further attempt to refer the phenomena of organogenesis to morphological and histological principles became desirable. More especially was the need felt to point out with greater minuteness and accuracy the relation in which the origin of the fundamental organs of the embryo stands to the layers of the blastoderm; and this we find accomplished with signal success in the researches of Eemak on the development of the chick and frog, published between the years 1850 and 1855. From Eemak s observations it appeared that the middle layer of the blastoderm, whatever may be the precise source from which it originally springs, a point left undetermined by Von Baer, Eemak, and even by more recent observers, becomes divided in its lateral portions into two laminaj, so as to leave between them the cavity which afterwards inter venes between the external wall and the contained viscera of the body. This cavity corresponds to the pleiiro-peri- toneal space of the higher animals, and may be designated in the lower by the general term of codom (Haeckel). While, therefore, Eemak recognized an outer and an inner layer of the blastoderm, corresponding only in some measure with the serous and mucous layers of Pander, he showed that the greater part of the middle layer is divided into two, the outer of which is the main source of the osseous and muscular walls of the body, and the inner is the seat of development of the involuntary contractile walls of the alimentary canal, the heart, and the principal vessels. Thus, according to the system of Eemak, while the central portion of the middle layer remains undivided, and gives rise to the axial chorda dorsalis or notochord, with the surrounding vertebral and cranial walls, the lateral parts of this layer are in the earlier stage of its develop ment split into two by the formation of the pleuro- peritoneal cavity, and there thus result the four layers whose relation may, according to the light received from more recent inquiry, be tabularly represented as follows : Primi- I tive ! lilasto- j derm Ectoderm Endoderm 1. Sensorial or Epiblasl ^ 2. Body Wall.... | Secon- 2. 3. Mesoderm I dary or Mesoblast.. | Blasto- 3. Visceral Wall derm. 4. Intestinal or Hypoblast J From the first of these layers (1), the neuro-corneous of Eemak, now named epiblast, the cuticular system and central organs of the nervous system (cerebro-spinal axis) are primarily formed, and secondarily, certain parts of the principal organs of sense, viz., the eye, ear, and nose. The motoro-germinative is the name applied to the middle layer by Eemak, of which (2), the outer division, the vohuito- motori/, corresponding to the body-wall or somatopleure of more recent authors, furnishes the material for the de velopment of the true skin, the voluntary muscles, and the skeleton ; and (3), the inner division, the involunto-motory, corresponding to the visceral wall or splauchno-pleure of recent authors, is the source of formation of the contractile wall of the alimentary canal, the heart, and larger blood vessels, the vascular glands, the primordial kidneys, and the generative organs. The fourth or lowest layer (4), the intestino-glandular of Eemak and the hypoblast of recent
writers, is the source of the epithelial lining of the alimen-