800 



GENERATION. 



hypoblastic spheres now passes to the centre of the ovum ; and the foui 

 epiblastic spheres, which are at the periphery, divide, each one into two, 

 making eight epiblastic and four hypoblastic spheres. When this has 

 occurred, the epiblastic spheres are .smaller and more transparent than the 

 hypoblastic spheres. The four hypoblastic spheres now divide into eight. 

 The epiblastic spheres then divide into sixteen, the hypoblastic spheres in 

 turn divide, and this goes on until the process of segmentation is com- 

 pleted. In the rabbit, this occurs usually about seventy hours after impreg- 

 nation (Van Beneden). As segmentation progresses, the epiblastic cells ex- 



FIG. 293. Formation of the blastodermic vesicle (Van Beneden). 



A, B, C, D, sections of ova in successive stages of development in the rabbit ; zp, zona pellucida ; 

 ep, epiblastic cells ; hyp, hypoblastic cells. 



tend over the hypoblastic cells, and become irregularly polygonal in form. 

 The hypoblastic cells occupy the central portion of the ovum. At first 

 there is a circular space upon the ovum where the epiblastic cells do not 

 cover the cells of the hypoblast (see Fig. 293, A) ; but this soon becomes 

 closed by an extension of the cells of the epiblast (see Fig. 293, B). The 

 hypoblastic cells, at the close of segmentation, are slightly larger than the 

 cells of the epiblast and are darker and more rounded. The ovum is now 

 called the morula, on account of its fancied resemblance to a mulberry; 

 and the cells of which it is composed are called collectively blastodermic 



