18 HEREDITY AND ENVIRONMENT 



oosperm, formed by the union of the two sex 

 cells, is really a double cell, since parts of the 

 egg and sperm never lose their identity, and 

 the individual which develops from this 

 oosperm is a double being; even in the adult 

 man this double nature, caused by the union of 

 egg and sperm, is never lost. 



In by far the larger number of animal 

 species the oosperm, either just before or 

 shortly after fertilization, is set free to begin 

 its own individual existence, and in such cases 

 it is perfectly clear that the fertilization of the 

 egg marks the beginning of the new individual. 

 But in practically every class of animals there 

 are some species in which the fertilized egg is 

 retained within the body of the mother for a 

 varying period during which development is 

 proceeding. In such cases it is not quite so 

 evident that the new individual comes into be- 

 ing with the fertilization of the egg; rather 

 the moment of birth or the separation from 

 the mother is generallj^ looked upon as the be- 

 ginning of the individual existence. And yet 

 in all cases the egg or embryo is always dis- 

 tinguishable from the body of the mother and 



