17S 



THE SEGMENT A T10X. 



At the moment of their first separation these spheres are spherical, 

 and arranged in two layers, one of them formed of the four epi- 

 blastic 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 constituted 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, together 

 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 seg- 

 mentation of both epiblastic and hypoblastic spheres continues, and 

 in the course of it the epiblastic spheres spread further and further 



\\ 



FIG. 134. OPTICAL SECTIONS -OF A RABBIT'S OVUM AT TWO STAGES CLOSELY 



FOLLOWING UPON THE SEGMENTATION. (After E. van Benedeu.) 



pp. epiblast; luj. primary hypoblast; l>p. Van Beneden's blastopore. 



The shading of the epiblast and hypoblast is diagrammatic. 



over the hypoblastic, so that at the close of segmentation the hypo- 

 blastic spheres constitute a central solid mass almost entirely sur- 

 rounded by the epiblastic spheres. In a small circular area however 

 the hypoblastic spheres remain for some time exposed at the surface 

 (fig. 134 A). 



The whole process of segmentation is completed in the rabbit 

 about seventy hours after impregnation. At its close the epiblast 

 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 Van 



