Germinal Layers in Vertebrates. 361 



different investigators with reference to these questions are 

 so very divergent as to furnish tlie best proof of the vague- 

 ness of the conceptions as to what is to be designated as 

 gastrulation. Just as many differences of opinion prevail on 

 the subject of tlie gastruLa mouth. According to the view of 

 certain enibryologists the blastopore in meroblastic ova corre- 

 sponds to the margin of the epibolic growth which surrounds 

 the yolk. Other enibryologists. on the contrary, insist that 

 the margin of the epibolic growth in no way corresponds to 

 the blastopore ; they maintain that it is a peculiarity of mero- 

 blastic ova, and so forth. It is proposed by this school to 

 designate as blastopore that spot in the germ at which an 

 invagination of cells takes place (in the Selachians the poste- 

 rior portion of the margin of the germinal disk, in the 

 Amniota the primitive streak and the primitive groove). 

 But this is not all. There are also embryologists who assume 

 that the blastopore is always turned towards the neural side 

 of the animal, where it closes up along a median line, which 

 is termed the gastrula suture (" Gastrularaphe ") . In this 

 manner, according to this conception, the nervous system 

 develops in the place of the gastrula suture, since the margins 

 of the blastopore become transformed into the medullary 

 folds ! 



In this brief communication it is indeed impossible for me 

 to enter into a discussion of all the theories wliich trace the 

 corresponding developmental processes of Vertebrates to 

 gastrulation, or which, in other words, would discover in 

 gastrulation a universal explanation of the formation of 

 germinal layers, as though no other processes could exist in 

 these stages. Many attempts have been made to apply the 

 gastrulation theory in the case of all Vertebrates, but all 

 these theories are in my opinion forced and unnatural. We 

 have yet to inquire whether it is not possible to interpret 

 these processes somewhat differently without unduly extending 

 the range of the theory of gastrulation. 



My investigations have led me to the conclusion that a 

 conception such as regards the process of invagination not as 

 gastrulation, but as a phenomenon which is characteristic of 

 all Chordata, is not merely possible, but is absolutely neces- 

 sary, if we would compare the earliest developmental pro- 

 cesses of different Vertebrates one with anotlier, and at the 

 same time retain the strict homology of the primary germinal 

 layers. I have studied the formation of the germinal layers in 

 the following animals : — Amj^hioxus and Petromyzon; AxolotJ^ 

 among the Amphibians ; Pristiurus and Torpedo among the 

 Selachians ; Lahrax^ Jtilis, and Gohius among Teleostean 



