296 



EVOLUTION AND GENETICS 



The spermatozoon, on the other hand, is highly differenti- 

 ated. It consists of several regions, particularly the head, neck 

 and tail. The head is a very compact nucleus, and the tail 



B 



Fig. 171. — Human germ cells. A, ovum, x 415, surrounded by follicle cells 

 from the ovary; B, sperm cell x 2000. (From Woodruff, after Retzius.) 



is an organ of locomotion. The human germ cells are shown in 

 Figure 171. 



Fertilization. This is the process of union of the two kinds of 

 germ cells. When a sperm cell comes into contact with an ovum 

 which has not previously been fertilized it penetrates the membrane 

 surrounding the ovum. The head, at least, enters the cytoplasm 

 of the ovum, while the tail remains outside. The surface of the 



