DEVELOPMENT OF THE FROG 123 



giving rise to an eight - celled stage, and the next divi- 



. 



X 





fc lt 



Fig. 26. 



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

 A, eight-cell stage resulting from two 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 yoke-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, ff, 

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

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

 blastula, showing the circular blastopore. 



sion is described as meridional. Each upper pigmented 

 blastomere is divided into two longitudinally, and the division 



