Xll 



DEVELOPMENT 



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

 cells — the yolk-plug, is left uncovered (Fig. 64, G, H, yk. pi). 



Development of tlie 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 



mdr 



mes 



KiG. 65. — Tratisv'cise hccliun LhruLigh :l frog-cniljryo during the formation 07 Lire 



medullary canal. 



c. cctlome ; ect. ectoderm ; end. endoderm ; cnt. enteron ; incs. mesoderm ; incs^. its 



outer (parietal) layer; tnns'^. its inner (visceral) layer; ind.f. medullary fold ; 



^nd.g): medullary groove ; itcii. notochord ; yk. yolk-cells. (After Marshall.) 



cavity, or archenteron (Fig. 64, I 2MA.()%eiif), 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 



