Germinal Layers in Vertebrates. 373 



of invagination as gaatnilation, for this process here bears no 

 relation to the formation of the intestine. It is true that 

 certain investigators describe a sac-hke invagination in the 

 case of Reptiles, and consider its cavity to be that of the 

 archenteron. But, in my opinion, this discovery does not 

 prove what these authors wish to prove, but rather exactly 

 the opposite, since it is evident from their description that, 

 although the cavity produced by invagination exists, never- 

 theless its Avails do not ])articipate in the formation of the 

 intestine, and that the cavity of invagination in no way 

 becomes that of the intestine. 



Consequently here also we see in the invagination a process 

 cliaracteristic of the Vertebrata, namely the formation of the 

 ectoblastogenous rudiment of the notochord and mesoderm. 

 In this case also the notochordal rudiment, after it has sepa- 

 rated from the lateral rudiments of the mesoderm, enters into 

 a temporary connexion with the endoderm cells, since it 

 becomes intercalated among them ; but subsequently the 

 notochord is once more excluded. A correct description of 

 this process has already been furnished by many investigators. 



As regards the mesoderm, we can here distinguish the axial 

 and peripheral regions of this layer. The axial mesoderm 

 develops on both sides of the notochord from the ectoblasto- 

 genous rudiment already described ; but the cells which split 

 off" from the endoderm also take part in the formation of these 

 mesodermal rudiments. The peripheral mesoderm is formed 

 from the endoderm cells. To the peripheral mesoderm like- 

 wise belongs that at the posterior margin of the primitive 

 streak, which also arises from the cells of the endoderm. 



The following important conclusions result from my inves- 

 tigations : — 



(1) In the invagination in the case of Amphioxus two 

 different processes are to be distinguished : — (a) The invagi- 

 nation ot the endoderm cells wdiich form the intestine (the 

 palingenetic process or gastrulation) ; {h) the dorsal invagi- 

 nation of the ectoderm cells, which constitutes the ectoblasto- 

 genous rudiment of the notochord and mesoderm (the ceno- 

 genetic process). 



(2) In all Vertebrates these two processes can be distin- 

 guished — the origin of the intestine from the endoderm cells, 

 and the formation of the rudiment of the notochord and 

 mesoderm from a proliferation of the cells of the ectoderm. 



Ann. <& Mag. N. Hist. Ser. 6. Vol. xi. 27 



