40 PHYSIOLOGICAL TRIGGERS 



still demands explanation. This is provided in part by the small proportion of 

 successful sperm-egg collisions. Calculating from the data of Rothschild and 

 Swann (89) at a sperm density of 10' per milliliter, 1000 sperm-egg collisions 

 should occur in a 63-second interval. On the average, 500 of these should occur 

 on the unblocked surface of the egg. Of these 500 collisions, 1.5 per cent, or 

 7.5, should be successful. 



This figure would admit of some polyspermy, but the low percentage of 

 successful collisions clearly helps to account for the absence of polyspermy 

 when 63 seconds are required for development of the complete block. A second 

 factor may also be involved in preventing polyspermy, namely a high speed, 

 but partial block. Evidence for the existence of such a block was derived from 

 consideration of the 'fertilization parameter,' a. The fertilization parameter is a 

 measure of the proportion of sperm-egg collisions that will succeed in fertilizing 

 the egg. It is, therefore, a measure of the 'fertilizability' of the gametes. Com- 

 parison of a for unfertilized and fertilized eggs during the 63-second interval 

 when the block to polyspermy is developing shows that a falls to about 1/20 

 of its prefertilization value following the first fertilization. In other words, 

 only 1/20 of the successful prefertilization collisions are successful following 

 the first fertilization. This suggests that a high speed but partial block sweeps 

 over the egg surface, followed by a slower, complete block. The high speed 

 block is estimated to require less than 2 seconds for completion, whereas the 

 low speed block is not complete until the end of the 63-second interval. This 

 postulated high speed, partial block, combined with the relatively low prob- 

 ability of a collision being successful, would seem to account for the absence 

 of polyspermy at fertilization. 



It should be noted that Allen and Hagstrom (4, 5) deny the existence of 

 the high speed block to polyspermy. Their argument is based on the observation 

 that sperm readily activate the unfertilized region of partially fertilized eggs. 

 However, in the absence of quantitative data it is impossible to evaluate the 

 position of Allen and Hagstrom. Furthermore, in partially fertilized eggs, 

 propagation of the cortical reaction and the complete block to polyspermy is 

 interrupted. This writer sees no reason why the postulated high speed block 

 may not also be interrupted by the treatment. In fact, it may be argued that 

 the experiments of Allen and Hagstrom support the high speed block. A high 

 speed block would be required to prevent polyspermy in material which nor- 

 mally fertilizes so readily. 



The studies of Rothschild and SAvann clearly show that the complete block 

 to polyspermy develops slowly in the normal egg. The means of propagation 

 of the block, and especially the rapid partial block postulated by these workers, 

 is unknown but some information concerning the mechanism of the block has 

 been obtained in recent years. 



It has long been known that treatment of eggs with agents such as nicotine, 

 chloroform and chloral hydrate facilitates polyspermy. With the exception of 



