Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/179

Rh E M D E M D 169 edited after his death by Kolliker in 1861, and the Lehrbuch dcr Verglcichemle Embryologie by S. L. Schenk, Vienna, 1874 We may also refer liere to the excellent plates illustrating embryology in the Iconcs physiological of A. Ecker, 1854. In this country, since the appearance of the very careful trans lation of Miiller s Physiology by Baly, which had the advantage of revision in many of its parts by Sharpey, and the translation of Wagner s Physiology in 1846, there has appeared only one syste matic work on embryology, viz., the .Elements of Embryology by M. Foster and F. M. Balfour, of which the first part, which appeared in 1874, treating of the development of the embryo of birds, deserves the highest praise. A short view of human embryology is given by Allen Thomson in the 8th edition of Quain s Anatomy, 1876. For an account of the relation of embryology to the classification of animals and to phylogeny or the theory of descent, the English reader is referred to various parts of the writings of Darwin and Huxley, and to the excellent translation of Haeckel s work on the History of Creation, 2 vols., London, 1874; to F. M. Balfour s &quot; Comparison of the Early Stages in the Development of Verte brates &quot; in Journ. of Microscopical Science, vol. xv., 1875, and to the recently published Notes on Embryology and Classification, by E. Kay Lankester, 1877. In the preceding sketch of the history of the foundation and progress of the science of embryology, no attempt lias been made to trace that part of it which includes the development of different invertebrate animals, as it was felt that from the extremely numerous, varied, scattered, and fragmentary nature of many of the contributions of authors in this part of our subject, any attempt at the citation even of the more important would be quite unsuitable to this work. It will be enough for us here to state that the first con siderable original work on the development of a division of the invertebrates was that of Maurice Herold of Marburg on spiders, &quot; De generatione Aranearum ex ovo,&quot; published at Marburg in 182-i, in which the whole phenomena of the formative process in that animal are described with remark able 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 Eutozoa was the production of Ch. Theod. Von Siebold, and that of most of the other invertebrates, w r as com piled by Rathke from the results of his own observations and those of others. These memoirs, together with others subsequently published by Rathke, entitle him to be re garded as the founder of invertebrate embryology. It would be quite unsuitable in this article to attempt to pursue further the history of research in the embryology of invertebrate animals, as may well bo seen from the fol lowing enumeration in an alphabetical order of the names of some of the more prominent original observers, to whom lias been mainly due the great progress in this part of our science, viz., Agassiz, Allman, Balbiani, Edward van Bene- den, P. J. van Beneden, Victor Carus, Claparede, Cohn, Dalyel), Darwin, Dujardin, Ecker, Eschricht, Gegenbaur, Haeckel, Huxley, Kolliker, Kowalewsky, Krohn, Lacaze Duthiers, Lereboullet, Leuckart, Leydig. Loven, Lubbock, MelschnikotF, Milne-Edwards, H. Miiller, Johannes Miiller, Nordmann, Prevost, Quatrefages, Salensky, Sars, Max Schultze, Semper, Steenstrup, Stein, C. Vogt, R. Wagner, Strethill Wright. But this list includes only a small part of the observers whose contributions to the knowledge of this wide field of research would require to be noticed in any account of its literature. The most general results which are deducible from the numerous observations which are now being accumulated in this department of embryology may be briefly stated as follows. In the Protozoa there is no true sexual generation, although in some the phenomena of conjugation form an approach to that nxode of reproduction. The greater number usually multiply either by fission or by gemmation; but in some, and probably in all, reproduction also appears to take place from extremely minute particles separated from the parent animals, which can scarcely be called ova. but which, for want of a better term, we may designate germinal particles. In all animals above the Protozoa, including the sponges, male and female reproductive elements are to be distin guished, that of the female taking the form of an ovum, in which the germinal part has the protoplasmic structure of a true organized cell. Fecundation of the ovum takes place, as in all verte brates, by the direct access of the substance of the male element to the germinal part of the ovum. The first steps in the development of a fecundated ovum are in all instances among the invertebrates, just as in the vertebrates, those of cellular multiplication by fission or cleavage of the protoplasmic germ of the ovum, which results in the formation of a more or less laminar blastoderm. This blastoderm presents at first two layers of cells, ectoderm and endoderm, in all animals above the Protozoa, and in the lowest of the Coalenterata only two ; but in all the higher animals there appears an additional intermediate layer or layers, constituting the mesoderm. From these layers the rudiments of the several systems and organs of the body are developed by processes of cel lular multiplication and differentiation according to certain histological and morphological laws essentially analogous to those which have been in part previously referred to in this article, and which will be more fully described in that on GENERATION. Having now traced the principal steps by which, upon the basis of extended morphological and histological ob servations during a century, extending from Wolff to Dar win, the science of embryology has been securely founded, enough has been adduced to show the important place which this science must occupy in relation to other depart ments of biology. It will be seen that histology owes to embryological observations the greatest amount of its recent extraordinary progress. It is also apparent that many of the most important facts in physiology, especially as related to growth and nutrition, can only be understood from a full and minute acquaintance with the various changes of differentiation observed in the development of organic structure. It is equally obvious that the nature of certain kinds of congenital malformation receive their rational explanation in the knowledge of the natural organogenetic process of development, from which they are no more than deviations in different modes and degrees. Nor can it be doubted that the arrangement of animals under an ap proved zoological system, in which the various affinities and gradations of their organization are fully recognized, can only be undertaken upon the basis of a complete knowledge of the metamorphoses of the young of animals and the relation of the embryonic to the adult forms of the species. Lastly, the general views which we may attempt to form of the process by which in the long lapse of time since the creation the various kinds of animals, including man, may be supposed to have originated mtibt be founded on the correlation of the ascertained facts of embryology, as observed in every animal species, with the fuller knowledge of the different forms and gradations of all existing animals, and with the more complete observa tion 01 the different forms of organization, the evidence of whose existence at successive periods of the earth s history is to be found in their fossil remains which are inclosed in the various strata composing its superficial crust. (A. T.) EM DEN, formerly EMBDEX, a maritime town of Prussia, in the district of Aurich, province of Hanover, is situated near the mouth of the Ems, on the Westphalian railway, VIII. 22