DEVELOPMENT OF FROG 



671 



Stage Roux's half-embryo regenerated the missing half, 

 usually by re-vitalising the remains of the cell which 

 was punctured. If the ovum be shaken about after 

 puncturing, a readjustment of material is effected, and a 

 half-sized embryo is formed (Morgan). The second 

 cleavage is also vertical, and at right angles to the first, 

 dividing an anterior from a posterior half. The third 

 cleavage is equatorial, at right angles to the first two, 

 dividing the dorsal region from the ventral. 



The segmentation is total but unequal, and results in the 

 formation of a ball of cells, those of the upper hemisphere 

 being smaller and more numerous than the yolk-laden cells 



1248 



Fig. 385. — Division of frog's ovum. — After Ecker. 

 The numbers indicate the number of cells or blastomeres. 



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 con- 

 tinue to divide much more rapidly than the large yolk- 

 containing cells, and so how the smaller ectodermal 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 

 horns of the crescent meet at a point near the lower pole 



