MATURATION 



still in the Graafian follicle. In the formation of the maturation spindle, 

 no astral rays and no typical centrosomes have been observed. The 

 chromosomes are V-shaped. The first polar cell is constricted from the 

 ovum and lies beneath the zona pellucida as a spherical mass about 

 25 micra in diameter (Fig. 13). Both ovum and polar cell (secondary 



a I J 



FIG. 13. Maturation and fertilization of the ovum of the mouse (after Sobotta). A, 

 C-J, X 500; B X 750. A -D, entrance of the spermatozoon and formation of the polar cells. 

 D-E, development of the pronuclei. F-J, successive stage in the first division of the fer- 

 tilized ovum. 



oocytes) contain 20 chromosomes, or half the number normal for the 

 mouse. The first maturation division is the reductional one and the 

 chromosomes take the form of tetrads. 



After ovulation has taken place, the ovum lies in the ampulla of the 

 uterine tube. If fertilization occurs, a second polocyte is cut off, the 

 nucleus of the ovum forming no membrane between the production of 



