FROM SIMPLE CELL TO COMPLEX ANIMAL. 31 



through which the spermatozoon finds entrance. Usually only 

 one sperm cell gains admission to the interior of the ovum, 

 whether by way of the micropyle or through the unmodified 

 membrane. Changes normally occur in the membrane as soon 

 as one sperm enters, by which all others are excluded. In 



FIG. 10. 



FIG. 10. Four stages in the maturation and fertilization of the ovum (partly dia- 

 grammatic). A, formation of the, second polar body and the entrance of the sperma- 

 tozoon; B, the male and female pronuclei, the former with aster about the centrosome; 

 C, nuclei coming together; sperm centrosome has formed two; D, pronuclei uniting to 

 form segmentation nucleus; asters producing spindle preparatory to cleavage, e.n., egg 

 nucleus; p.b., polar bodies; s, spermatozoon; s.c., sperm centrosome and aster; s.n., 

 sperm nucleus; se, segmentation nucleus produced by the union. 



Questions on the figure. In what respect is the formation of the polar 

 bodies similar to ordinary indirect cell-division? In what respects different 

 from it? Is there any difference shown in the figures between the first 

 and the second polar bodies? In what, apparently, does maturation con- 

 sist? In what way does fertilization appear to compensate for the loss in 

 the formation of the polar bodies? 



eggs which have been kept too long or subjected to unfavor- 

 able conditions, the response of the membrane may not be so 

 quickly effected and multiple fertilization may occur. Such 

 fertilizations may produce monstrosities. The sperm nucleus 

 is now called the male pronudens. It migrates toward the 



