AMPHIBIA 



297 



egg is called the gastrula. The upper germ layer or ectoblast de- 

 velops from this layer of black cells. 



From the development of the band of cells and the fusion of its 

 right and left halves, a meridional band extends into the dorsal lip 

 of the blastopore. On the dorsal side, the groove becomes a long 

 narrow slit which is the primitive digestive tract of the frog. The 

 roof of the niesenteron, as it is called, is the beginning of the lower 

 germ layer, the entoblast. 



Fig. \b\A. Vinal Edwards and Robert Goffin at the nets. Woods Hole, Mass. 



The middle germ layer, the mesoblast^ arises as two plates split 

 off from the outer surface of the entoblast and yolk cells. The 

 mesoblast separates into two layers with a space between which 

 becomes the coelom. 



Germ Layers. — From the ectoblast are derived epidermis, nerv- 

 ous system and the lining of the mouth and anus. An anterior 

 invagination called the stomodaeum develops into the mouth and 

 a posterior one into the proctodaeum. 



From the entoblast are derived the lining epithelium of the diges- 

 tive tract and its associated glands. 



From the mesoblast come muscles, bones, blood vessels and urino- 

 genital organs. 



Superficial Changes. — At first the egg is spherical, then in four 

 or five days it becomes ovoid, finally elongating at about the loth 



