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 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 segmentation of both epiblastic and hypoblastic spheres continues, and in the course of it the epiblastic spheres spread further and further over the hypoblastic, so that at the close of segmentation the hypoblastic spheres constitute a central solid mass almost entirely surrounded 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 Beneden the blastopore, though it is highly improbable that it in any way corresponds with the blastopore of other vertebrate ova[76]. [76] It is stated by Bischoff that shortly after impregnation, and before the commencement of the segmentation, the ova of the rabbit and guinea-pig are covered with cilia and exhibit the phenomenon of rotation. This has not been noticed by other observers. [FIG. 134. OPTICAL SECTIONS OF A RABBIT'S OVUM AT TWO STAGES CLOSELY FOLLOWING UPON THE SEGMENTATION. (After E. van Beneden.) _ep._ epiblast; _hy._ primary hypoblast; _bp._ Van Beneden's blastopore. The shading of the epiblast and hypoblast is diagrammatic.]