244 COMPARATIVE ZOOLOGY OF CHORDATES 



the archenteron to the surface of the yolk. The walls of the 

 definitive alimentary canal are formed from the lower layer, 





c?.* 



t, r Jk. 



Ct.fe.Jk. 



c?J. 



Fig. 115. — Longitudinal sections through the blastoderm of a reptile, 

 showing the origin of the blastopore. (From Jenkinson, after Will.) 

 A-E, successive stages. A, precocious origin of the endoderm or lower 

 layer (pd), which is in contact with the upper layer of the blastoderm at 

 pp ; B, origin of the blastopore, dl, dorsal lip of the blastopore (cf. Fig. 89) ; 

 C, invagination at the blastopore to form the archenteron ; the lower layer 

 forms a continuous membrane separated from the yolk by the subgerminal 

 cavity (sgc) ; mesv, mesoderm formed from the lip of the blastopore ; D, 

 the archenteron (arch) extends a long way forwards beneath the upper layer 

 of the blastoderm ; yp, yolk-plug (cf. Fig. 75) ; E, fusion of the floor of 

 the archenteron with the underlying region of the lower layer, and their 

 subsequent disappearance, so that the archenteron communicates with the 

 subgerminal cavity. 



which is endoderm formed really before the blastopore proper 

 can be said to exist. 



