FERTILIZATION 



33 



becomes resolved into the reduced number of chromosomes which it re- 

 ceived during maturation (Fig. 24, F). A centrosome with astral radia- 

 tions is now seen between the two groups. In Fig. 24, G, it has divided 

 in two, and the spindle has developed. There has been much discussion 

 as to the origin of these centrosomes. Since in this case they arise by the 

 division of a single body, the possibility that one comes from the sper- 



H ' J 



FIG. 34. MATURATION AND FERTILIZATION OF THE OVUM OF THB MOUSE .A,C-J,Xsoo; 8X750. 



(After Sobotta.) 



A-C, entrance of the spermatozoon and formation of the second polar body. _ D-E, development 

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



matozoon and one from the ovum has been eliminated. Moreover in the 

 mouse they cannot be derived from the surviving centrosome of the last 

 maturation division of the ovum, for that division takes place without 

 centrosomes. Therefore the centrosome must either be brought in by the 

 spermatozoon as a constituent of its middle piece, or it must be a new forma- 

 tion. Sobotta accepted the former alternative, and he observed a centro- 



3 



