6oo 



AMPHIBIA. 



below. Within there is a small segmentation cavity. Since 

 the presence of yolk acts as a check on the activity of the 

 protoplasm, we can understand why the smaller cells continue 

 to divide much more rapidly than the large yolk-containing 

 cells, and so how the smaller epiblastic cells gradually 

 spread over the egg, covering in the larger ones. At one 

 point, where upper and lower cells meet, a groove is 

 formed. This groove represents the dorsal lip of the 

 blastopore. It becomes crescentic and moves as a whole 

 down over the large yolk-cells. Invagination of the small 

 cells of the upper hemisphere goes on rapidly all round this 

 crescentic groove, and the archenteron is thus formed. The 

 cr rp horns of the cres- 



N tL cent meet at a point 



near the lower pole 

 of the egg to form 

 the ventral lip of 

 the blastopore. 

 The blastopore 

 now becomes re- 

 duced, by the in- 

 growing of its mar- 

 gins, to a small 

 circular area which 

 appears white, the 

 colour being due 

 to a plug of yolk- 

 cells which almost 

 obliterates its 



FIG. 325. Longitudinal vertical section of 

 frog embryo, shortly before closure of blasto- 

 pore. After Ziegler's model and Marshall. 



FB., fore-brain; C., ectoderm; N., notochord ; SC., 

 canal of spinal cord; NE. t neurenteric canal; B. y 

 blastopore; M., mesoderm cells; K., Yolk-laden 

 cells ; MN., mesonteron ; P., beginning of pituitary 

 invagination. 



opening. The whole egg now rotates backwards through 

 a little more than a right angle, so that the blastopore is 

 carried up into the position previously occupied by the first 

 trace of its dorsal lip. The blastopore now marks the 

 posterior end of the embryo. The archenteron has by this 

 time greatly enlarged, and has pushed the segmentation cavity 

 almost out of existence. The embryo elongates slightly, but 

 the mass of yolk-laden cells which lie on the floor of the gut 

 prevents the body acquiring at once the fish-like shape. 



Along the mid-dorsal line the usual neural plate forms the 

 medullary canal. At the posterior end this communicates 

 with the archenteron for a time by the neurenteric canal. 



