DEVELOPMENT OF THE FROG 123 



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





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Fig. 26. 

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

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

 divisions. B, beginning of si.xteen-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. £!, F, two sides of the same ovum 

 in later stages of segmentation. C, a still later stage of segmentation, /f, 

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

 crescehtic line bl^ 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 



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