92 THE MAMMALIAN EGG 



The act of sperm penetration is thought to depend on an enzyme 

 or similar agent associated with the perforatorium in the sperm head 

 which is exposed when the acrosome is detached (Austin and Bishop, 

 1958a, b, c). Although a number of points of indirect evidence 

 favour the involvement of a lytic agent in sperm passage through 

 the zona pellucida, and analogous mechanisms are known in 

 invertebrates, no success has yet been obtained in attempts to extract 

 such an agent from mammalian spermatozoa. It is possible that the 

 hypothetical zona lysin is active only while attached to the per- 

 foratorium. Dauzier and Thibault (1956) report that uterine 

 polymorphonuclear leucocytes enter eggs in culture; since it is 

 conceivable that the mechanism of penetration is similar, investiga- 

 tions on this problem might profitably include study of these cells. 



Study of the numbers of spermatozoa entering the eggs of rats 

 and mice showed that the zona pellucida could reasonably be held 

 to undergo a change after the entry of the first spermatozoon which 

 tended to exclude other spermatozoa, and this change was termed 

 the zona reaction (Braden, Austin and David, 1954). The zona 

 reaction is thus a mechanism, like the block to polyspermy, that 

 helps to prevent the occurrence of polyspermic fertilization. In the 

 rat, the mean time the reaction takes to reach completion was 

 estimated to be not less than 10 min nor more than ij to 2 hr. In 

 the rat, mouse, guinea-pig, cat and ferret, the reaction may be 

 classed as moderately efficient — though the number of spermatozoa 

 that pass through the zona is limited, it is not merely the fertilizing 

 spermatozoon that is successful, and eggs are often seen in which 

 one or, less commonly, a few supplementary spermatozoa are 

 present in the perivitelline space, excluded from the vitellus by the 

 block to polyspermy. By contrast, supplementary spermatozoa are 

 rarely if ever to be found in the perivitelline space of the eggs of 

 the hamster, field vole, dog and sheep, and in these animals the 

 reaction may be classed as highly efficient. At the other extreme, 

 the eggs of the rabbit (see Adams, 1955), pika (Harvey, 1958) and 

 mole (Heape, 1886) appear to lack a zona reaction for they regularly 

 have quite large numbers of supplementary spermatozoa, the rabbit 

 egg often as many as 200 or 300. The eggs of the pocket gopher, 

 with 'several' to 'numerous' spermatozoa in the perivitelline space 

 (Mossman and Hisaw, 1940), presumably have a very slow reaction. 



The best explanation of the mechanism of the zona reaction 

 seems to be that attachment of the fertilizing spermatozoon to the 



