C. R. AUSTIN AND M. W. H. BISHOP 93 



environment and its vagaries. With internal fertilization there is 

 a much greater likelihood of success, which is achieved through 

 the deposition of the gametes into a highly specialized internal 

 environment. The success of fertilization in this environment, as 

 with the outcome of many other biological situations, is deter- 

 mined by the results of interaction between opposing functions: 

 between those responsible for activating the spermatozoa and 

 those responsible for protection against overactivation, between 

 those favoring rapid transport of spermatozoa and those hinder- 

 ing transport, between those promoting spermatozoon penetra- 

 tion of egg membranes and those preventing excessive penetra- 

 tion. The most important variables appear to be the number of 

 spermatozoa reaching the site of fertilization and their time of 

 arrival. The many integrated processes and reactions that consti- 

 tute the environment of fertilization in mammals have the func- 

 tion of ensuring the meeting of the gametes in the most appro- 

 priate numerical relations and at the optimal phase of their life 

 cycle. In no two species, however, is the mechanism quite the 

 same, so that the preliminaries to fertilization among different 

 species exhibit wide variations in the degree of development of 

 the constituent complementary processes. 



There may also be disadvantages to internal fertilization which 

 stem from the fact that biological systems are often highly spe- 

 cific within the individual, but they tend to differ between indi- 

 viduals. In particular the immunological processes behave in this 

 way. Spermatozoa are known to show a variety of antigenic prop- 

 erties ( Smith, 1949a,b ) , including the possession of blood group 

 antigens appropriate to the male producing them (Docton, Fer- 

 guson, Lazear, and Ely, 1952 ) . Owing to antigenic diversity be- 

 tween individuals, spermatozoa within the female tract may, to 

 varying degree, be regarded as foreign bodies. According to the 

 "self-not-self" concept, it is a fundamental property of adult 

 organisms that foreign material can be recognized as such and 

 rejected (Burnet and Fenner, 1949; Burnet, 1954). A mechanism 

 of this nature may underlie the failure of heterologous spermato- 

 zoa in many abortive attempts at hybridization. It is possible, 

 also, that antigenic incompatibility between the spermatozoa and 



