55° 



AMPHIBIA. 



was punctured. If the ovum be shaken about after punctur- 

 ing, 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 

 below. Within there is a small segmentation cavity. Since 

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



Fig. 236. — Division of frog's ovum. — After Ecker. 

 The numbers indicate the number of cells or blastomeres. 



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 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. According to the older view, at this point the 

 small cells are invaginated, and so form a cavity ; according 

 to recent research, the cavity is simply formed by the split- 

 ting of the large cells. However this may be, the cavity, 

 which is the archenteron or embryonic gut, rapidly enlarges 

 at the expense of the segmentation cavity, which soon dis- 

 appears. The groove becomes a circular aperture in the 

 epiblast, which has now spread over the whole egg except 



