EMBRYOLOay 23 



forniini!; a j^astruhi. The ()i)t'ninti; at tlic ])()iiit where the invagina- 

 tion of \egetal cells occurred is called the hld.sioporc. The outer 

 layer of cells conijHJses the ectoderm ; the iiuier layer is the endoderm; 

 and the cavity within the endoderm layer of cells is the archenterun, 

 or primitive gut, which oi)ens to the exterior by the blastopore. 



A third germ layer, the mesoderm, makes its initial appearance 

 from a series of ])aired i)ouch-like evaginations along each side of 

 the dorso-lateral wall of the endoderm of the primitive gut. These 

 evaginations, the coelomic pouches, later separate from the endo- 

 derm. The ctrlomic ])()uches on either side enlarge and grow ven- 

 trally around the gut, the transverse walls of adjacent pouches fuse 

 and are ()l)literated so that a connnon cavity, the coelome, is formed. 

 The outer wall of the mesoderm adjacent to ectoderm is known as 

 somatic mesoderm and together with the ectoderm is called the 

 somat()i)leure, which gives rise to the bodv wall. The inner hiver 



Ectoderm- 

 i Primitive gut --Wp^Qs^.^'^SSyt^ „ , 



Endoderm 



8 CELL STAGE BLASTULA GASTRULA 



Fig. 3. — Diagram of early stages in development of the frog. 



of mesoderm adjacent to the endoderm of the gut wall is known as 

 splanchnic mesoderm and together with the endoderm composes 

 splanchnopleure, which gives rise to the gut wall. 



Amphibia. — In this group, as exemplified by the frog, the eggs 

 contain considerable yolk. The nucleus and bulk of the cyto])lasm 

 are restricted to the animal pole and the abundant yolk is concen- 

 trated toward the \'egetal pole. Cleavage is complete but the cells 

 are of unequal size, those toward the vegetal ])ole being distinctly 

 larger than those of the animal i)ole. The segmentation cavity of the 

 blastula is reduced in size with yolk-laden cells on its floor. (Fig. 3.) 

 Gastrulation is atypical, invagination of vegetal cells being limited, 

 and the gastrula appears to be formed as a result of overgrowth 

 and infolding of the more actively dividing cells of the animal pole 

 in the region of the blastopore. The gastrula is similar to that of 

 Ami)hioxus, excei)t for the larger yolk-laden cells below the floor of 

 the archenteron. 



