282 PROGRESS OF MIOROSCOPIOAL SCIENCE. 



Kleinenberg, and some others, have resulted in extending to the entire 

 auimal kingdom this grand conception that all the parts of the animal 

 organism are formed from the two primordial cellular layers, and 

 everywhere homologous. 



" These ideas have just been developed in detail and brilliantly 

 defended in two essays of a high philosophic import. Haeckel has 

 proposed in his brochure ' Die Gastrjea theorie, die phylogenetische 

 Classihcation des Thierreiches und die Homologie der Keimbliitter,' 

 a theory which he had first announced in his monogragh on the cal- 

 careous sponges. Some analogous ideas, and in several respects 

 almost identical, have been published in England in the Annals and 

 Magazine of Natural History, under the title, " On the Primitive 

 Cell-layers of the Embryo as the Basis of the Genealogical Classifica- 

 tion of Animals," by my friend E. Kay Lankester. 



" All the pluricellular animals, in which the development begins 

 by the segmentation of the cell-egg, pass through in the com-se of 

 their evolution a similar embryonic form, that of a sac whose thin 

 walls are constituted of two adjacent layers, the endoderm and ecto- 

 derm. The first surrounds a cavity which is the primordial digestive 

 tube ; the second limits exteriorly the body of the embryo ; it alone 

 can be impressed by external causes. The digestive cavity commu- 

 nicates with the exterior by a single orifice which serves both as 

 mouth and anus. The embryo is reduced to a digestive cavity, which 

 is but a simple stomach; Haeckel has proposed to give to this 

 primordial form the name of Gasirula. As this embryonic form 

 occurs in the vertebrates as well as the mollusks, arthropods, echino- 

 derms, worms and polypes, it is clear that the ectoderm is homologous 

 in the different types of organization ; that the endoderm has in all 

 the same mori^hological value ; that the primordial digestive cavity of 

 vertebrates and that of all other types of organization have the same 

 anatomical signification. The existence of this common form in the 

 course of evolution of all the metazoal animals allows us to refer 

 them to a common source ; there is a convergence of the great 

 types of organization and not a parallelism as had been urged by 

 Cuvier and Von Baer. Finally, we can infer the existence at a 

 geological epoch far back, of organisms like the Gastrula form; 

 these organisms, probably varied in a thousand ways in their form and 

 in their external characters, have been the common source of verte- 

 brates, arthropods, mollusks, echinoderms, worms, and zoophytes ; 

 they constitute the very numerous group of Gastrseades (Haeckel). If 

 the endoderm and ectoderm are homologous in all the Metazoa [i. e. 

 all animals except Protozoa], we then have a right to suppose that 

 these two cellular layers have in all the same histological value, and 

 that the same systems of organs are developed in the different types 

 of organization from the same primitive layers. This induction 

 has been already freely confirmed in that which concerns the cen- 

 tral nervous system, which is developed in all animals from the 

 ectoderm. 



" Consequently, it makes no difference if we should wish to know 

 the origin of an organ, whether we seek for it in one or another type 



