Page:Anatomy of the Human Body (1918).djvu/41

Rh distributed between the nuclei of the two cells formed by the first division of the ovum. One of the cells is almost as large as the original ovum, and is named the secondary oöcyte; the other is small, and is termed the first polar body. The

Fig. 4.—Formation of polar bodies in Asterias glacialis. (Slightly modified from Hertwig.) In I the polar spindle (sp) has advanced to the surface of the egg. In II a small elevation (pb$1$) is formed which receives half of the spindle. In III the elevation is constricted off, forming the first polar body (pb$2$), and a second spindle is formed. In IV is seen a second elevation which in V has been constricted off as the second polar body (pb$1$). Out of the remainder of the spindle (f.pn in VI) the female pronucleus is developed.

secondary oöcyte now undergoes subdivision, during which each dyad divides and contributes a single chromosome to the nucleus of each of the two resulting cells. Priviary oöcyte

Fig. 5.—Diagram showing the reduction in number of the chromosomes in the process of maturation of the ovum.

This second division is also unequal, producing a large cell which constitutes the mature ovum, and a small cell, the second polar body. The first polar body frequently divides while the second is being formed, and as a final result four cells