C. R. AUSTIN AND M. W. H. BISHOP 91 



consequently the zona reaction commences at or near the site of 

 penetration and progressively involves the rest of the zona. 



In the hamster the vitellus of the impenetrated egg displays a 

 large number of cortical granules that disappear when the head 

 of the fertilizing spermatozoon makes contact with the surface 

 (Austin, 1956b). Light scatter by the vitelline surface also dimin- 

 ishes after spermatozoon contact. There is good evidence that a 

 vitelline block to polyspermy does not operate in hamster eggs 

 (Austin, 1956c) so that the cortical changes seen are very likely 

 associated with the zona reaction which is well developed in 

 hamster eggs. 



In certain respects the responses shown by mammalian eggs 

 are similar to processes involved in membrane elevation in echi- 

 noderm eggs. The fertilization membrane is considered to be the 

 vitelline membrane modified by the products of a change propa- 

 gated through the cortex from the point of spermatozoon contact. 

 The numerous cortical granules, which "explode" when the sper- 

 matozoon enters the egg, evidently play a part in the modification 

 of the vitelline membrane (Moser, 1939; Motomura, 1941; Runn- 

 strom, Monne, and Wicklund, 1944, 1946; Runnstrom and Wick- 

 lund, 1950; Endo, 1952). Changes in the light-scattering prop- 

 erty of the cortex have also been described, although there is 

 disagreement on the direction of the change and on its possible 

 association with the disappearance of cortical granules or the for- 

 mation of the block to polyspermy ( Moser, 1939; Rothschild and 

 Swann, 1949). 



The zona reaction and the vitelline block to polyspermy are 

 complementary in function. In some species, such as rats and 

 mice, both are operative. In others, such as the hamster, sheep, 

 and dog, protection appears to be vested exclusively in the zona 

 reaction. In others again, such as the rabbit and perhaps the mole 

 and the pocket gopher, the zona reaction is slow or absent and 

 the block to polyspermy seems to be unusually well developed 

 (Braden, Austin, and David, 1954). 



The block to polyspermy evidently involves a change in the 

 vitelline surface such that attachment will no longer be formed 

 with the head of a contacting spermatozoon (Austin and Braden, 



