xii DEVELOPMENT 201 



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

 cells the yolk-filug, is left uncovered (G, H, yk. pl\ 



Development of the 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 



mes 



enl 



FIG. 65. Transverse section through a frog-embryo during the formation of the 



medullary canal. 



c. coelome ; ect. ectoderm ; end. endoderm ; ent. archenteron ; mes. mesoderm ; 

 mes*. its outer (parietal) layer ; mes*. its inner (visceral) layer ; md.f. medullary 

 fold; md.gr. medullary groove; nch. notochord ; yk. yolk-cells. (After 

 Marshall.) 



cavity, or archenteron (Figs. 64, i and 65, ent\ 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 



