MATTTRATION 



20 



(According to Mark and Long, the chromosomes number 20.) The first matura- 

 tion 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 takes place, a second polocyte is cut off, the nucleus of the 



E I J 



Fig. 13. — Maturation and fertilization of the ovum of the mouse. A, C-J, X 500; B X 750. (after 

 Sobotta). A-C, entrance of the spermatozoon and formation of the second polar body. D-E, develop- 

 ment of the pronuclei. F-J, successive stages in the first division of the fertilized ovum. 



ovum forming no membrane between the production of the first and second polar 

 bodies (Fig. 13 A-D). The second maturation spindle and second polar body are 

 smaller than the first Immediately after the formation of the second polar 

 body, the chromosomes resolve themselves into a reticulum and the female pro- 

 nucleus is formed (Fig. 13 D). 



