360 HeiT B. Lwoft'ou the Fvrmation of the 



LX. — On the Formation of the Gertninal Layers in Verte- 

 brates. By BasiliuS Lwoff, of Moscow University*. 



Extensive comparative investigations upon the formation of 

 the germinal layers in Vertebrates have led me to conclude 

 that all the theories at present prevailing on the subject of 

 gastrulation and germinal-layer-formation in this division of 

 the animal kingdom are not in accordance with the facts. 

 All these theories regard the invagination, which is charac- 

 teristic for all Chordata, as nothing else than gastrulation, 

 although in the majority of cases this invagination boars no 

 relation to the formation of the intestine. Tlie consequence 

 of this is that in the higher Vertebrata we have nothing to 

 help ns to distinguish the primary germinal layers. Starting 

 from the conviction that the true endoderm is always formed 

 by invagination, authors have proceeded so far in this direc- 

 tion as to regard the lower layer in the case of tlic Amniota, 

 from which the intestine is formed and which is homologous 

 with the endoderm of the lower Vertebrates, not as the endo- 

 derm, but as something that admits of no homology with 

 what is found in the lower Vertebrates (paraderm &c.) ; as 

 endoderm, however, are designated the ectoderm cells which 

 are invaginated or grow inwards, but take no ]iart in the 

 formation of the intestine. Or, again, it is assumed that both 

 the cells of the lower layer, from which the intestine is 

 formed, and the invaginated ectoderm cells are to be con- 

 sidered as endoderm ; but the lower layer, from which the 

 intestine arises, is designated secondary or cenogenetic endo- 

 derm, while to the invaginated cells, which represent the 

 rudiment of the notochord and mesoderm, the term ])rimary 

 or palingenetic endoderm is applied. It therefore follows 

 according to tliis interpretation that the intestine is formed 

 from the cenogenetic and the notochord from the palingenetic 

 endoderm. In order to realize how untenable this concep- 

 tion is we need only ask which is pliylogenetically the older, 

 the intestine or the notochord ? 



Since we would reg.ird the process of invagination which 

 is characteristic of all Vertebrates as gastrulation in its 

 original simplicity, it is natural that we should desire to 

 discover a gastrula mouth in these supposed gastrulas also, 

 and to determine in all Vertebrates the homology of the 

 dorsal and ventral lips of the blastopore. But the views of 



• Tranelated from the • Biologiscbes Centralblatt,' xiii. I3d., no. 2, 

 Feb. ], ]H9.'i, pp. 40 .50 ; and no. .S, Feb. 15, 189.3, pp. 70-81. 



