XII DEVELOPMENT 201 



small circular space — the blastopore, filled by a mass of yolk- 

 cells — ih.^ yolk-plug, is left uncovered (Fig. 64, G, Yi., yk.pt). 



Development of ttie Chief Organs. — In the meantime 

 a second cavity appears in the interior of the embryo, and 

 as it increases in size, the segmentation cavity undergoes a 

 proportional diminution and finally disappears. This new 



m.d.c 



Tnes 



ent 



Fig. 65. — Transverse section through a frog-embryo during the formation oi the 



medullary canal. 



c. coslome ; ect. ectoderm ; end. endoderm ; ent. enteron ; tnes. mesoderm ; mes^. its 



outer (parietal) layer ; ntes^. its inner (visceral) layer ; nzd.f. medullary fold ; 



7nd.gr. medullary groove ; nch. notochord ; yk. yolk-cells. (After Marshall.) 



cavity, or archenteron (Fig. 64, 1 and 65, en{), is the rudiment 

 of the enteric canal : it begins to be formed as a narrow 

 slit at the edge of the blastopore through which it at 

 first communicates with the exterior, but it soon becomes 

 closed : at present there is neither mouth nor anus. 



By this time the cells of which the embryo is composed 

 have assumed different forms and become arranged in a 



