DEVELOPMENT OF SPELEKPES BILINEATUS 



203 



The section just mentioned is the 61st from the blastopore end of the 

 series, in a total of 106 sections. It is not quite vertical, the ventral 

 part lying posterior to a vertical plane, the dorsal part, anterior. The 

 figure represents the condition of the entire roof of the archenteron 

 except in the immediate region of the blastopore, where the archenteron 

 is bounded dorsally and laterally by rather small cells, as might be ex- 

 pected from an examination of fig. 33. 



Figs. 36-39 



Not until the archenteric pouch has reached its greatest exten- 

 sion and the cleavage cavity become practically obliterated can 

 we definitely separate the yolk cells of the lateral roof of the 

 archenteron into mesoderm and endoderm. The former is a 

 layer of small cells (fig. 37), usually two deep, but not j'-et form- 

 ing well-defined layers. It extends from near the mid-dorsal 

 region to near the level of the floor of the archenteron, a. The 



