discovery in 1847 of the process of partial yolk segmentation in the germinal disk 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 elucidation from the researches of Rathke, in his history of the development of serpents, published at Königsberg 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 connexion 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 observations of Kölliker in the same animals in 1843. The nature of partial segmentation of the yolk was first made known by Kölliker in his work on the development of the Cephalopoda 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, Götte and Kölliker. 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.
The first considerable work on the development of a division of the invertebrates was that of Maurice Herold of Marburg on spiders, De generatione aranearum ex ovo, published at Marburg in 1824, in which the whole phenomena of the formative processes in that animal are described with remarkable clearness and completeness. A few years later an important series of contributions to the history of the development of invertebrate animals appeared in the second volume of Burdach’s work on Physiology, of which the first edition was published in 1828, and in this the history of the development of the Entozoa was the production of Ch. Theod. von Siebold, and that of most of the other invertebrates was compiled by H. Rathke from the results of his own observations and those of others. These memoirs, together with others subsequently published by Rathke, notably that Über die Bildung und Entwickelungsgeschichte d. Flusskrebses (Leipzig, 1829), in which an attempt is made to extend the doctrine of the derivation of the organs from the germinal layers to the invertebrata, entitle him to be regarded as the founder of invertebrate embryology.
A large body of facts having by this time been ascertained with respect to the more obvious processes of development, 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 R. Remak on the development of the chick and frog, published between the years 1850 and 1855.
Starting from Pander’s discovery of the trilaminate blastoderm, Remak worked out the development of the chick in the light of the cell-theory of Schleiden and Schwann. He observed the division of the middle layer into two by a split which subsequently gives rise to the body-cavity (pleuro-peritoneal space) of the adult; and traced the principal organs which came from these two layers (Hautfaserblatt and Darmfaserblatt) respectively. In this manner the foundations of the germ-layer theory were established in their modern form.
A great step forward was made in 1859 by T. H. Huxley, who compared the serous and mucous layers of Pander with the ectoderm and endoderm of the Coelenterata. But in spite of this comparison it was generally held that germinal layers similar to those of the vertebrata were not found in invertebrate animals, and it was not until the publication in 1871 of Kowalewsky’s researches (see below) that the germinal layer theory was applied to the embryos of all the Metazoa. But the year 1859 will be for ever memorable in the history of science as the year of the publication of the Origin of Species. If the enunciation of the cell-theory may be said to have marked a first from a second period in the history of embryology, the publication of Darwin’s great idea ushered in a third. Whereas hitherto the facts of anatomy and development were loosely held together by the theory of types which owed its origin and maintenance to Cuvier, L. Agassiz, J. Müller and R. Owen, they were now combined into one organic whole by the theory of descent and by the hypothesis of recapitulation which was deduced from that theory. First clearly enunciated by Johann Müller in his well-known work Für Darwin published in 1864 (rendered in England as Facts for Darwin, 1869), the view that a knowledge of embryonic and larval histories would lay bare the secrets of race history and enable the course of evolution to be traced and so lead to the discovery of the natural system of classification, gave a powerful stimulus to embryological research. The first fruits of this impetus were gathered by Alexander Agassiz, A. Kowalewsky and E. Metschnikoff. Agassiz, in his memoir on the Embryology of the Starfish published in 1864, showed that the body-cavity in Echinodermata arises as a differentiation of the enteron of the larva and so laid the foundations of our present knowledge of the coelom. This discovery was confirmed in 1869 by Metschnikoff (“Studien üb. d. Entwick. d. Echinodermen u. Nemertinen,” Mém. Ac. Pétersbourg (7), 41, 1869), and extended by him to Tornaria, the larva of Balanoglossus in 1870 (“Untersuchungen üb. d. Metamorphose einiger Seethiere,” Zeit. f. wiss. Zoologie, 20, 1870). In 1871 Kowalewsky in his classical memoir, entitled “Embryologische Studien an Würmern und Arthropoden” (Mém. Acad. Pétersbourg (7), 16, 1871), proved the same fact for Sagitta and added immensely to our knowledge of the early stages of development of the Invertebrata. These memoirs formed the basis on which subsequent workers took their stand. Amongst the most important of these was F. M. Balfour (1851–1882). Led to the study of embryology by his teacher, M. Foster, in association with whom he published in 1874 the Elements of Embryology, Balfour was one of the first to take advantage of the facilities for research offered by Dr. A. Dohrn’s Zoological Station at Naples which has since become so celebrated. Here he did the work which was subsequently published in 1878 in his Monograph of the Development of Elasmobranch Fishes, and which constituted the most important addition to vertebrate morphology since the days of Johannes Müller. This was followed in 1879 and 1881 by the publication of his Treatise on Comparative Embryology, the first work in which the facts of the rapidly growing science were clearly and philosophically put together, and the greatest. The influence of Balfour’s work on embryology was immense and is still felt. He was an active worker in every department of it, and there are few groups of the animal kingdom on which he has not left the impress of his genius.
In the period under consideration the output of embryological work has been enormous. No group of the animal kingdom has escaped exhaustive examination, and no effort has been spared to obtain the embryos of isolated and out of the way forms, the development of which might have a bearing upon important questions of phylogeny and classification. Of this work it is impossible to speak in detail in this summary. It is only possible to call attention to some of its more important features, to mention the more important advances, and to refer to some of the more striking memoirs.
Marine zoological stations have been established, expeditions have been sent to distant countries, and the methods of investigation have been greatly improved. Since Anton Dohrn founded the Stazione Zoologica at Naples in 1872, observatories for the study of marine organisms have been established in most countries. Of journeys which have been made to distant countries and which have resulted in important contributions to embryology, may be mentioned the expedition (1884–1886) of the cousins Sarasin to Ceylon (development of Gymnophiona),
