THE EARLY DEVELOPMENT OF THE MAMMAL 377 



large chromatin reservoir or karyosome. It is now ready to 

 enter upon the period of maturation. As a rule, in the Mam- 

 mals, the first polar body is given off while the ovum is still 

 within the ovary (in the mouse about one hour before ovula- 

 tion), and the second polar spindle is established; the second 

 polar division is then completed only after fertilization, which 

 occurs, of course, in the oviduct. It is not necessary to describe 

 the details of the maturation process here, for in most respects 

 it offers nothing unusual. With respect to the size of the polar 

 spindles and polar bodies, however, the Mammals are rather 

 remarkable, for these, especially the first polar body, are very 

 large, often one-fourth the diameter of the ovum itself, and in 

 some cases even larger. Occasionally an abnormally large 

 first polar body may be formed so that the egg divides almost 

 equally; the later history of such cases is not known. The 

 first polar body usually divides soon after its formation; 

 amoeboid movements have been observed in the first polar body 

 of the mouse. Centrioles are distinctly present, but the asters 

 are only slightly indicated or absent. 



A large second polar spindle is formed at once and moves 

 toward the surface of the egg (secondary oocyte,) when the 

 process of maturation is inhibited while the processes of ovula- 

 tion and fertilization occur. The second polar body is then 

 formed while the ovum is in the upper part of the oviduct; it 

 is smaller than the first. The polar bodies do not remain 

 attached to the surface of the ovum (Fig. 147) and are easily 

 lost sight of entirely. The number of chromosomes can be 

 most easily determined during these phases; some of the 

 somatic numbers determined are the following: man, twenty- 

 two in the male, twenty-four in the female (Guyer), mouse 

 twenty-four, cat between twenty-eight and thirty-two. 



4. Ovulation 



The escape of the ovum (secondary oocyte) from the Graafian 

 follicle and the ovary, is termed ovnlation. The capsule of the 

 maturing follicle becomes very vascular, and at one place very 



