POLYSPERMY 



III 



25 second period was terminated. This experiment provides strong 

 evidence for the conduction time of the block to polyspermy being 

 of the order of seconds rather than small fractions of a second, as 

 used to be thought. At the same time there remains the question 

 as to why, if the block to polyspermy takes seconds to pass over 

 the egg surface, so few eggs are normally polyspermic even at 

 comparatively high sperm densities. This difficulty can be resolved 

 by experiments based on the following self-evident proposition. 

 Suppose, for example only, that a number of eggs are all fertilized at 

 t = o and that the block to polyspermy is complete at t == 5 seconds. 



TABLE 17 

 Conduction velocity of block to polyspermy {Rothschild & Swann, 



1951) 



After 5 seconds there will be a a certain number of polyspermic 

 eggs in the egg population. The eggs will not all be polyspermic 

 because some of them will not have sustained more than one suc- 

 cessful sperm-egg collision during that 5 seconds. As, however, 

 all the blocks to polyspermy are complete by t = 5, the number of 

 polyspermic eggs will never be greater than it is at t = 5. If the 

 percentage of polyspermic eggs is 50 after 5 seconds, the per- 

 centage will still be 50 after 6, 60, or 600 seconds. It follows from 

 this argument that, if we take a series of egg suspensions and fer- 

 tilize all the eggs in them at t = o, and then 'remove' the sperm- 

 atozoa (Rothschild & Swann, 1951; Hagstrom & Hagstrom, 

 i954«) from these suspensions at various times after t = o, for 

 example, at 5, 10, 15 and 40 seconds, the time after which there is 

 no increase in the incidence of polyspermy will be the conduction 

 time of the block to polyspermy. Conversely, any decline in the 

 incidence of polyspermy at t = r, as compared with t = s, where 

 r > s, will be due to sampling errors or mistakes in deciding 



