88 PRELIMINARIES TO FERTILIZATION 



upon the zona. Adherence of the spermatozoon to the zona pro- 

 vides a parallel in these respects to the trapping of invertebrate 

 spermatozoa in the jelly coat by the fertilizin-antifertilizin reac- 

 tion (Tyler, 1948). It has indeed been shown that there is evi- 

 dence for this reaction in mammals: spermatozoa of the rabbit, 

 mouse, and bull were found to undergo agglutination in vitro in 

 the presence of homologous eggs but showed little effect in the 

 presence of eggs of the other species (Tyler, 1954; Bishop and 

 Tyler, 1956). These authors believe that a fertilizin analogous to 

 that of sea urchin eggs exists in the zona pellucida and that this 

 reacts with antifertilizin on the spermatozoon head so as to bring 

 about both the attachment of the spermatozoon to the zona 

 and its passage through the membrane. On the other hand, ad- 

 herence of the spermatozoon to the zona may well depend upon 

 the same mechanism as head agglutination. If this is so, the ant- 

 agglutin described by Lindahl and his associates would oppose 

 spermatozoon penetration and there would need to be something 

 present about the egg or at the site of fertilization with the prop- 

 erty of counteracting the antagglutin. 



Following attachment to the zona pellucida, the fertilizing 

 spermatozoon passes through this membrane leaving a small slit 

 that can be recognized many hours after penetration in the eggs 

 of rats (Austin, 1951) and some other rodents. Passage through 

 the zona is evidently rapid, at least in rats, mice, and hamsters, 

 for fertilizing spermatozoa are rarely seen with their heads still 

 within the thickness of the zona ( Austin and Braden, 1956 ) . The 

 possession by the spermatozoon of an agent capable of exerting 

 a lytic action upon the substance of the zona is a reasonable sup- 

 position, but so far no direct evidence of such an agent has been 

 advanced. As already noted, spermatozoa require to spend a pe- 

 riod in the female genital tract in order to undergo capacitation, 

 which may therefore involve either the addition to the sperma- 

 tozoon of a lytic agent produced in the female tract or the acti- 

 vation in the tract of a lytic agent carried by the spermatozoon 

 in an inactive form. A priori, the latter alternative seems prefer- 

 able (Austin, 1951). 



In varying forms, mammalian spermatozoa possess a structure 



