TYPES OF CHORDATE BLASTULAE 



365 



ARCHENTERIC SPACE 



TRO PHOBL AST 



ARCHENTERIC SPA CE 



Fig. 178. Schematic drawings of the developing blastocyst of the monkey. (After 

 Heuser and Streeter: Carnegie Inst., Washington. Publ. 538. Contrib. to Embryol. No. 

 181.) (A, B) Early blastocysts showing formative and non-formative cells in the inner 

 cell mass. (C-E) Later arrangement of the formative cells into an upper epiblast and 

 lower hypoblast layer. 



These cells form the mother entoderm cells, and by mitotic activity they give 

 origin to an entodermal layer which adheres to the underside of the thickened 

 polar area (fig. 176B, C). The polar area then thins out to form the expansive 

 condition shown in figure 176D. A bilaminar, disc-shaped area thus is formed 

 in this immediate region of the blastocyst, and it represents the area occupied 



