72 Specificity in Fertilization 



normal. It is purely due to limitation of food that 

 bacteria or certain species of them do not cover the 

 whole planet. But, as every layman knows, the major- 

 ity of organisms grow only to a certain size, then die, 

 and the propagation takes place through sex cells or 

 gametes: a female cell the egg containing a large 

 bulk of protoplasm (the future embryo) and reserve 

 material; and the male cell which in the case of the 

 spermatozoon contains only nuclear material and no 

 cytoplasmic material except that contained in the tail 

 which in some and possibly many species does not enter 

 the egg. The male element the spermatozoon enters 

 the female gamete the egg and this starts the de- 

 velopment. In the case of most animals the egg cannot 

 develop unless the spermatozoon enters. The question 

 arises: How does the spermatozoon activate the egg? 

 And also how does it happen that the spermatozoon 

 enters the egg? We will first consider the latter ques- 

 tion. These problems can be answered best from ex- 

 periments on forms in which the egg and the sperm are 

 fertilized in sea water. Many marine animals, from 

 fishes down to lower forms, shed their eggs and sperm 

 into the sea water where the fertilization of the egg 

 takes place, outside the body of the female. 



The first phenomenon which strikes us in this con- 

 nection is again a phenomenon of specificity. The sper- 

 matozoon can, as a rule, only enter an egg of the same 

 or a closely related species, but not that of one more 



