38 Jenkinson, Germinal Layers of Vertebrates. 



plug {Fig- 22, C, D). The resemblance of a section 

 through the blastopore to a section througli the circular 

 blastopore of a frog or newt is in fact complete. If the 

 section were continued, however, through the entire egg 

 a very striking discrepancy would be at once apparent, a 

 discrepancy which depends on the difference in the mode 

 of formation of the ventral lip. 



In other Amphibia the ventral lip is developed from 

 that point in the subequatorial transition zone which is 

 diametrically opposite to the dorsal lip, and the whole of 

 the vegetative surface of the egg becomes consequently 

 covered up when the blastopore closes. Here, on the 

 contrary, the anterior edge of the blastoderm and a large 

 part of the lateral edges have no share whatever in the 

 ventral lip, which arises entirely by the fusion of the 

 extremities of the two lateral lips ; as a result the vegeta- 

 tive hemisphere remains entirely uncovered. This is 

 shown very well in Sarasin's figure of IcJitJiyopJiis 

 {Fig. 21, E). In other words while in the Anura and Uro- 

 dela (and all other Anamnia) the whole of the edge of the 

 blastoderm becomes converted into a blastoporic lip, the 

 posterior point being the dorsal, the anterior becoming 

 sooner or later the ventral lip, in the Gymnophiona only 

 a small posterior region of the blastoderm's margin ever 

 becomes active in this way, and this small portion gives 

 rise to the dorsal and the two lateral lips, which latter by 

 their fusion produce such a remarkable similitude of the 

 ventral lip of the other forms. The importance of this 

 fact for the correct understanding of the relations of the 

 blastopore to the blastoderm in the Amniota cannot 

 possibly be overestimated. 



To return to the germinal layers. The superficial 

 layer of the blastoderm is now of course the ectoderm. 

 The plate of cells which forms the roof of the archenteric 



