94 



THE MAMMALIAN EGG 



The following observations support the suggestion that the 

 mammalian zona reaction belongs to this general series of reactions: 

 (a) In rat eggs penetrated by two spermatozoa, the slits left in the 

 zona pellucida were more often in opposite hemispheres than in the 



Fig. 72 

 Diagrams of a sea-urchin egg to show how the cortical granules are 

 considered to react to sperm contact with the vitellus and take part in 

 the elevation of the fertilization membrane. 



same one, a distribution that points to a propagated reaction (Braden, 

 Austin and David, 1954). (b) Unfertilized mouse eggs with 

 perivitelline spermatozoa well past the time of fertilization have 

 been observed after heat treatment of eggs (Austin and Braden, 

 1956) and in a certain inbred strain of mice (Krzanowska, i960); in 

 both instances, attachment of the sperm head to the vitelline surface 

 had evidently failed and in both instances the zona reaction had 

 failed also, for the number of perivitelline spermatozoa was much 

 higher than is seen in normally fertilized eggs, (c) In one mammal 

 at least, the golden hamster, cortical granules exist which disappear 

 following sperm contact with the vitellus (p. 65). The inferred 

 relationship between the zona reaction and the cortical-granule 



