l82 



ESSENTIALS OF BIOLOGY 



the cytoplasm of the ovum (Fig. 25, i). The chromosomes of 

 these pronuclei may be separately visible, or their materials may 

 be dispersed as a '' reticulum." Next the separate pronuclei 

 lose their sharpness by dissolution of their membranes. The 



Fig. 25. — Diagram of the Phases in a Typical Process of Fertilization 



OF AN Ovum. 



chromosomes take on definite shape and come to lie near each 

 other. Then they are arranged in a " plate " occupying the 

 '* equatorial " region of the fertilized ovum (Fig. 25, 2). Each 

 chromosome splits lengthwise (Fig. 25, 4) and the half-chromo- 

 somes (24 now) lie in an approximate plane near the " equator " 

 of the egg-cell. These half- chromosomes draw, or are drawn, 

 apart towards each pole of the egg-cell. The egg-cell now divides 

 into two daughter cells and further divisions occur as the ovum 

 segments (Section 70^). Schematically the process is as follows : 



Sperm nucleus 

 A 

 B 

 C 

 D 

 E 

 F 



+ 



Egg nucleus 

 a 

 b 

 c 

 d 

 e 

 f 



Nucleus of 

 fertilized ovum 

 A 

 B 

 C 

 D 

 E 

 F 

 a 

 b 

 c 



de 

 f 



