166 EMBRYOLOGY also the interesting commentary on or supplement to the jEpislola in Heusinger s Journal, 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 Entwiclcelungsgeschichte der Thiere, the second part of which, still leaving the work incomplete, did not appear till 1838. In this work, distinguished by the fulness, richness, and extreme accuracy of the observations and descriptions, as well as by the breadth and soundness of the general views on embryology and allied branches of biology which it presents, he gave a detailed account not only of the whole progress of development of the chick as observed day by day during the incubation of the egg, but he also described what was known, and what he himself had investigated by numerous and varied observations, of the whole course of formation of the young in other vertebrate animals. His work is in fact a system of comparative embryology, replete with new discoveries in almost every part. Von Baer s account of the layers of the blastoderm differs somewhat from that of Pander, and appears to be more consistent with the further researches which have lately been made than was at one time supposed, in this respect, that he distinguished from a very early period two primitive or fundamental layers, viz., the animal or upper, and the vegetative or lower, from each of which, in connec tion with two intermediate layers derived from them, the fundamental organs and systems of the embryo are derived: the animal layer, with its derivative, supplying the der mal, neural, osseous, and muscular ; the vegetative layer, with its derivative, the vascular and mucous (intestinal) systems. He laid down the general morphological principle that the fundamental organs have essentially the shape of tubular cavities, as appears in the first form of the central organ of the nervous system, in the two mus cular and osseous tubes which form the walls of the body, and in the intestinal canal; and he followed out with admirable clearness the steps by which from these funda mental systems the other organs arise secondarily, such as the organs of sense, the glands, lungs, heart, vascular glands, Wolftian bodies, kidneys, and generative organs. To complete Von Baer s system there was mainly wanting a more minute knowledge of the intimate structure of the elementary textures, but this had not yet been acquired by biologists, and it remained for Thomas Schwann of Lie"ge in 1839, along with -whom should be mentioned those who, like Robert Brown and Schleiden, prepared the way for his great discovery, to point out the uniformity in histological structure of the simpler forms of plants and animals, the nature of the organized animal and vegetable cell, the cellular constitution of the primitive ovum of animals, and the derivation of the various textures, complex as well as simple, from the transformation or, as it is now called, differentiation of simple cellular elements, discoveries which have exercised a powerful and lasting influence on the whole progress of biological knowledge in our time, and have contributed in an eminent degree to promote the advance of embryology itself. To Reichert of Berlin more particularly is due the first application of the newer histological views to the explanation of the phenomena of development, 1840. To him and to Kolliker and Virchow is due the ascertainment of the general principle that there is no free-cell formation in embryonic development and growth, but that all organs are derived from the multiplication, combination, and transformation of cells, and that all cells giving rise to organs are the descendants or progeny of previously existing cells, and that these may be traced back to the original cell or cell- substance of the ovum. It may be that modern research has somewhat modified the views taken by biologists of the statements of Schwann as to the constitution of the oganized cell, espe cially as regards its simplest or most elementary form, and has indicated more exactly the nature of the protoplasmic material which constitutes its living basis; but it has not caused any very wide departure from the general principles enunciated by that physiologist. Schwann s treatise, entitled Miscrocopical Researches into the Accordance in the Structure and Growth of Animals and Plants, was published in German at Berlin in 1839, and was translated into English by Henry Smith, and printed for the Sydenham Society in 1847, along with a translation of Schleiden s memoir, " Contributions to Phylogenesis," which originally appeared in 1838 in Miiller s Archiv for that year, and which had also been published in English in Taylor and Francis s Scientific Memoirs, vol. ii. part vi. Among the newer observations of the same period which contributed to a more exact knowledge of the structure of the ovum itself may be mentioned first, the discovery of the germinal vesicle, or nucleus, in the germ -disk of birds by Purkinje (Symbolce ad ovi avium historiam ante incubationem, Vratislavice, 1825, and republished at Leipsic in 1830); second, Von Baer s discovery of the mammiferous ovum in 1827, already referred to; third, the discovery of the germinal vesicle of mammals by Coste in 1834, and its independent observation by Wharton Jones in 1835; and fourth, the observation in the same year by Rudolph Wagner of the germinal macula or nucleus. Coste s discovery of the germinal vesicle of Mammalia was first communicated to the public in the Comptes Rendus of the French Academy for 1833, and was more fully described in the Recherches sur la generation des Mammiferes, by Delpech and Coste, Paris, 1834. Thomas Wharton Jones s observations, made in the autumn of 1834, without a knowledge of Coste s communication, were presented to the Royal Society in 1835. This discovery was also confirmed and extended by Valentin and Bernardt, as recorded by the latter in his work Symb. ad ovi Mammal, hist, ante prcp.gnationem. Rudolph Wagner s observations first ap peared in his Textbook of Comparative Anatomy, published at Leipsic in 1834-5, and in Miiller s Archiv for the latter year. His more extended researches are described in his work Prodromus hist, gtnerationis hominis atque animaliiim, Leipsic, 1836, and in a memoir inserted in the Trans, of the Roy. Bavarian Acad. of Sciences, Munich, 1837. The two decades of years from 1820 to 1840 were peculiarly fertile in contributions to the anatomy of the foetus and the progress of embryological knowledge. The researches of Prevost and Dumas on the ova and primary stages of development of Batrachia, birds, and mammals, made as early as 1824, deserve especial notice as important steps in advance, both in the discovery of the process of yolk segmentation in the batrachian ovum, and in their having shown almost with the force of demon stration, previous to the discovery of the mammiferous ovarian ovum by Von Baer, that that body must exist as a minute spherule in the Graafian follicle of the ovary, although they did not actually succeed in bringing the ova clearly under observation. The works of Pockels (1825), of Seiler (1831), of Breschet (1832), of Velpeau (1833), of Bischoff (1834) all bearing upon human embryology ; the researches of Coste in comparative embryology in 1834, already referred to, and those published by the same author in 1837 ; the publication of Joannes Miiller s great work on physiology, and Rudolph Wagner s smaller text-book, in both of which the subject of embryology received a very full treatment, together with the excellent Manual of the Development of the F&tus, by Valentin, in 1835, the first
separate and systematic work on the whole subject, now