MAMMALIA. 215 



other. The larger sphere and its products will be spoken of as 

 the epiblastic spheres, and the smaller one and its products as 

 the hypoblastic spheres, in accordance with their different 

 destinations. 



Both the spheres are soon divided into two, and each of the 

 four so formed into two again; and thus a stage with eight 

 spheres ensues. At the moment of their first separation these 

 spheres are spherical, and arranged in two layers, one of them 

 formed of the four epiblastic spheres, and the other of the four 

 hypoblastic. This position is not long retained, but one of the 

 hypoblastic spheres passes to the centre; and the whole ovum 

 again takes a spherical form. 



In the next phase of segmentation each of the four epiblastic 

 spheres divides into two, and the ovum thus becomes consti- 

 tuted of twelve spheres, eight epiblastic and four hypoblastic. 

 The epiblastic spheres have now become markedly smaller than 

 the hypoblastic. 



The four hypoblastic spheres next divide, giving rise, to- 

 gether with the eight epiblastic spheres, to sixteen spheres in 

 all; which are nearly uniform in size. Of the eight hypoblastic 

 spheres four soon pass to the centre, while the eight superficial 

 epiblastic spheres form a kind of cup partially enclosing the 

 hypoblastic spheres. The epiblastic spheres now divide in their 

 turn, giving rise to sixteen spheres which largely enclose the 

 hypoblastic spheres. The segmentation of both epiblastic and 

 hypoblastic spheres continues, and in the course of it the epi- 

 blastic spheres spread further and further over the hypoblastic, 

 so that at the close of segmentation the hypoblastic spheres con- 

 stitute a central solid mass almost entirely surrounded by the 

 epiblastic spheres. In a small circular area however the hypo- 

 blastic spheres remain for some time exposed at the surface (fig. 

 1 34 A). 



The whole process of segmentation is completed in the rabbit 

 about seventy hours after impregnation. At its close the epi- 

 blast cells, as they may now be called, are clear, and have an 

 irregularly cubical form ; while the hypoblast cells are polygonal 

 and granular, and somewhat larger than the epiblast cells. 



The opening in the epiblastic layer where the hypoblast cells 

 are exposed on the surface may for convenience be called with 



