58 Jenkinson, Germinal Layers of Vertebrates. 



making the Gymnophionan correspond to the whole of 

 the Amphibian blastopore, and in returning to Rabl's 

 erroneous conception of the ventral augmentation of the 

 yolk. 



So far then no theory has been propounded which, in 

 my opinion at any rate, affords an adequate explanation 

 of the facts. And yet the explanation is obvious enough. 

 In the Gymnophiona [Fig. 33, C), as we have seen 

 already, 



(i) The blastoderm is a circular area of columnar 

 cells resting upon and surrounded by a partially 

 segmented yolk. 



(2) At the posterior margin of this blastoderm a 



dorsal lip is formed and lateral lips quickly 

 follow ; the lateral lips then grow back encir- 

 cling a small area of the yolk behind which they 

 meet and fuse to form a ventral lip to the now 

 circular blastopore. In this process the anterior 

 margin of the blastoderm is wholly unconcerned. 



(3) The archenteron which is formed at the blasto- 



poric lip, opens into the segmentation cavity. 



(4) Notochord and mesoderm are derived from the 



ingrowth at the lip of the blastopore, while the 

 endoderm arises from the yolk-cells. 



And now let us describe the same stages in the 

 development of a Reptile i^Fig. 33, D). 



(i) The embryonic shield is a circular area of 

 columnar cells resting upon a paraderm and 

 surrounded by a zone of flattened cells. 



(2) At the posterior margin of this embryonic shield 

 a dorsal lip is formed and lateral lips quickly 

 follow ; the lateral lips then grow back encircling 



