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EMBRYOLOGY 



larger sphere subsequently forms, by its division, the epiblastic cells, 

 and the smaller sphere forms the hypoblastic cells. Each sphere is 

 provided with a distinct nucleus. The two spheres resulting from the 

 first segmentation l are divided, each one into two, making four spheres. 



\ 



Fig. 221. Later singes of segmentation of the ovum of a bat (van Beneden). 

 A, C and D are sections ; B, a surface view. 



These spheres are again divided into eight four epiblastic and four 

 hypoblastic spheres each with a nucleus (van Beneden). One of the 

 four Hypoblastic spheres passes to the centre; and the four epiblastic 



1 In the mammalia, segmentation, after the first division of the ovum into two, is irregular, 

 " three-celled, four-celled, five-celled and six-celled stages having been observed in various 

 instances" (McMurrich). 



