238 



GENERAL ZOOLOGY 



whose metabolism has almost come to equilibrium. Eggs requir- 

 ing fertilization die unless fertilized. 



The spermatozoon is a very small fraction of the size of the 

 egg, even if the egg has but little yolk. In sharp contrast with 

 the immobility of the egg, the sperm is usually capable of ener- 



/ 

 Memh 



-o 



Fig. 145. — A nearly ripe human ovum in the living condition. The ovum 

 is surrounded by follicle cells [FC) inside of which is the clear membrane (Memb) 

 and within this is the ovum proper containing yolk granules (F) and a nucleus 

 (N) embedded in a clear mass of cytoplasm. Magnified 500. B, two human 

 spermatozoa drawn to about the same scale of magnification. (From Conklin, 

 Heredity and Environment, Princeton University Press. After 0. Hertwig; B after 

 G. Retzius. By permission.) 



getic movements by means of its flagellum or tail. In some forms, 

 such as nematode worms and crustaceans, a tail is lacking and the 

 sperm is incapable of rapid locomotion (Fig. 146). Spermatozoa 

 vary greatly in structure. In motile forms, the sperm consists of 

 two general regions, (1) the head, which proceeds first in locomo- 

 tion and which contains the chromatin, and (2) the tail, which is a 

 stout flagellum, sometimes provided with a fin. Between the 

 head and tail, a neck or middle-piece region may be recognized. 



