14 HEREDITY AND ENVIRONMENT 



eggs of oysters and worms, which may be no 

 more than a few thousandths of a millimeter 

 in diameter. The human ovum (Fig. 1) is 

 microscopic in size (about 0.2 mm. in diameter) 

 but it is not smaller than is found in many 

 other animals. It has all the characteristic 

 parts of any egg cell, and can not be distin- 

 guished microscopically from the eggs of sev- 

 eral other mammals, yet there is no doubt that 

 the ova of each species differ from those of 

 every other species, and later we shall see 

 reasons for concluding that the ova produced 

 by each individual are different from those 

 produced by any other individual. 



The sperm, or male gamete, is among the 

 smallest of all cells and is usually many thou- 



... T ... M H 



A --'""" "****- ''' 



A _____ a* 



B 



FIG. 2. Two HUMAN SPERMATOZOA. A, showing the surface 

 of the flattened head; B, its edge; H, head; M, middle-piece; 

 T, tail. (After G. Retzius.) 



