126 COMPARATIVE ANATOMY 



The ovum of the frog, as has been said, is abundantly 







Fig. 27. 



Segmentation of the frog's ovum and formation of the blastopore (after Morgan). 

 A, eight-cell stage resulting from wo meridional and one equatorial 

 divisions. B, beginning of sixteen-cell stage. C, Thirty-two-cell stage. 

 D, Forty-eight-cell stage, showing the smaller cells at the upper pole and 

 larger yolk-laden cells at the lower pole. E, F, two sides of the same ovum 

 in later stages of segmentation. G, a still later stage of segmentation. //, 

 the smaller cells are growing over the larger cells but are involuted along a 

 crescentic line />/, the dorsal lips of the blastopore. /, external view of the 

 blastula, showing the circular blastopore. 



provided with food-yolk, stored up chiefly in one hemisphere 

 of the cell. The first segmentation following upon fertilisa- 



