36 Jenkinson, Germinal Layers of Vertebrates. 



The dorsal lip quickly grows back, and produces a long 

 archenteron, which at its anterior end opens into the 

 segmentation cavity. The roof of the archenteron, 

 which seems to be derived entirely from the superficial 

 layer of the blastoderm, consists of a plate of columnar 

 cells, its floor of the partially segmentedyolk (^Fig. 22, B). 



Fig. 21. 

 Gymnophiona. 

 A — D. Four successive stages in the closure of the blastopore and 

 formation of the ventral lip in Hypogeophis (after Brauer). The blastoderm 

 only is shown ; y.p. yolk -plug. 



E. Diagrammatic section of the egg of Ichthyophis (after Sarasin) to 

 show the embryo resting on a partially segmented and unenclosed yolk. 



The process of overgrowth is of course not limited to 

 the dorsal lip, but extends to the immediate right and 

 left. Surface views show that the transverse groove, the 

 outward sign of this lip, soon becomes crescentic ; the 

 horns of the crescent then grow not only backwards, but 

 towards the middle line as well, approaching one another 



