84 PRELIMINARIES TO FERTILIZATION 



Chang, 1951a, 1955a,b). In the rat penetration did not occur 

 until four or more hours later. In the rabbit few or no eggs were 

 fertilized, although control tests, in which spermatozoa were in- 

 troduced before ovulation, yielded a high incidence of fertiliza- 

 tion. The failure of fertilization in the rabbit was explained on 

 the grounds that the eggs became impenetrable, through the 

 deposition of the mucoprotein layer, before the spermatozoa had 

 completed capacitation. Spermatozoa obtained from the uterus 

 of an inseminated rabbit showed the ability to penetrate eggs 

 after a shorter interval of time. Noyes ( 1953 ) found, by insemina- 

 tion into the uterus, that rat uterine spermatozoa penetrated eggs 

 after a shorter time than did epididymal spermatozoa. It ap- 

 peared, therefore, that, while capacitation could take place 

 entirely in the tubes, it could also occur, to some extent at least, 

 in the uterus. The possibility that the effects observed had been 

 due in part to the operative procedures employed was ruled out 

 by the demonstration of a delay between the arrival of the sper- 

 matozoa at the site of fertilization and the penetration of the 

 eggs in intact rats and rabbits when coitus occurred after ovula- 

 tion (Austin, 1952; Austin and Braden, 1954a). It is uncertain 

 what capacitation involves, but it seems to take at least two hours 

 in the rat and four hours in the rabbit, Capacitation is not simply 

 the separation of the spermatozoa from the male accessory secre- 

 tions, for epididymal spermatozoa must also spend a period within 

 the female tract. It is not associated with any obvious change in 

 the structure of the spermatozoa. The most likely explanation 

 seems to be that capacitation involves the activation of an enzyme 

 system; this may possibly be effected by interaction with some- 

 thing originating in the follicular secretions. 



The Chances of Fertilization 



It is common experience that, when eggs are recovered from 

 animals such as rats and mice early in the day following coitus, 

 few or no spermatozoa can be seen in the cumulus mass or in the 

 free surrounding fluid, and yet all the eggs are found to have 

 been penetrated. The circumstances certainly suggest the exist- 

 ence of chemotaxis between egg and spermatozoon. Chemotaxis 



