POLYSPERMY II5 



time is 10 minutes; but the implications of this inefficiency depend 

 on the sperm density in the neighbourhood of the eggs and, there- 

 fore, on the sperm-egg colHsion frequency. If the colHsion fre- 

 quency is low, a fast block to polyspermy is unnecessary and a 

 simple calculation shows that if we assume an average sperm speed 

 of I GO /x. /sec. and the value o-oi for the probability of a successful 

 sperm-egg collision, a sperm density of the order of ^/fA, in the 

 neighbourhood of the eggs, would not be excessive. But why 

 should a mammalian egg need two distinct blocks to polyspermy ? 



An alternative interpretation of the experiments is worth con- 

 sideration. In a number of non-mammalian eggs the cortical block 

 to polyspermy is associated with the outward diffusion of substances 

 into the perivitelline space, where they react with and tan the 

 vitelline membrane (which is not, of course, always homologous 

 with the zona). These substances are progressively released from 

 the egg, following the progressive change in the egg cortex known 

 as the block to polyspermy. It follows that the vitelline membrane 

 is progressively tanned, though it is not excited, by the penetrating 

 spermatozoon. The tanning is a passive process which results 

 from the excitation, by a spermatozoon, of the egg cortex. 



There are the following arguments against this hypothesis and 

 in favour of the zona being capable of propagating its own tanning 

 reaction in the rat egg : 



(i) Cold-shock induces contraction of the egg and the release 

 of fluid into the perivitelline space, both of which are charac- 

 teristic of activation ; but the zona remains permeable to sperm- 

 atozoa (Austin & Braden, 19546). 



(2) Heat-shock inhibits the cortical block to polyspermy, but 

 has little effect on the zona reaction (Austin & Braden, 19546). 



(3) After penetrating the zona, spermatozoa may remain for 

 as long as thirty minutes in the perivitelline space before fertiliz- 

 ing the egg (Austin & Braden, 1954a). There should, therefore, 

 be little relationship between the point of penetration through 

 the zona and the point of attachment or fertilization on the egg 

 surface. 



The last argument seems the most cogent; but a more direct 

 confirmation of the existence of this zona reaction would be wel- 

 come. 



Induced Polyspermy. The possibility of interfering with the 



