90 PRELIMINARIES TO FERTILIZATION 



of penetration, were seen much more commonly when the semen 

 used for insemination had been treated with a nonspecific hyalu- 

 ronidase inhibitor (Parkes, Rogers, and Spensley, 1954). In mice 

 subjected to hyperthermia, eggs with spermatozoa only in the 

 perivitelline space were chiefly those showing early induced par- 

 thenogenesis (Austin and Braden, 1956). Evidently, artificial ac- 

 tivation of mouse eggs is apt to prevent attachment of spermato- 

 zoon to vitellus. 



In brief, the passage of the spermatozoon through the cumulus 

 and zona pellucida is presumed to be made possible by enzymes 

 that it carries, whereas entry into the cytoplasm of the egg ap- 

 pears largely to be an active function of the vitellus. Attachment 

 of the spermatozoon to the zona and then to the vitellus must 

 evidently precede penetration of these membranes. 



Immediate Reactions of the Egg 

 to Spermatozoon Penetration 



In most mammals — a notable exception being the rabbit — the 

 zona pellucida undergoes a change after the entry of the first 

 spermatozoon, with the result that further spermatozoa tend to 

 be excluded (Braden, Austin, and David, 1954; earlier observa- 

 tions on the incidence of supplementary spermatozoa in mamma- 

 lian eggs are reviewed in this paper ) . This is known as the "zona 

 reaction," and the rate at which it occurs varies between species 

 so that a few extra spermatozoa are often seen in the eggs of the 

 rat, mouse, guinea pig, cat, and ferret, but rarely if ever in those 

 of the dog, sheep, and hamster. In the rat the reaction takes be- 

 tween 10 minutes and 2 hours to reach completion. Smithberg 

 (1953) reported that, in mice, the zona pellucida is removed by 

 proteolytic enzymes more rapidly from unfertilized than from 

 fertilized eggs. This supports the idea that the zona reaction 

 renders the substance of the zona refractory to the action of the 

 supposed zona lysin. The change appears to be induced by an 

 agent released from the vitellus following attachment of a sper- 

 matozoon to its surface (Austin and Braden, 1956). The release 

 of the agent is evidently propagated through the cortex of the 

 egg from the point of attachment of the spermatozoon head, and 



