CHARLES B. METZ 39 



number of sperm-oocyte collisions (less than i bleb for every 45 collisions). 

 Accordingly, if the oocytes are unable to propagate a block to polyspermy 

 and the blebs faithfully record successful sperm-egg collisions, the latter must 

 be but a small proportion of the total collisions. Using mature eggs, this view- 

 was further confirmed. Eggs were inseminated with sperm suspensions of 

 known densities and at intervals the excess sperms were killed (by hypotonic 

 sea water treatment); the proportion of fertilized eggs was then determined 

 and the number of sperm-egg collisions calculated. From this information the 

 probability of a successful collision, />, was calculated for a number of sperm 

 densities. Aside from the interesting observation that increasing the sperm 

 density decreased the value of />, j)resumably because of sperm-sperm inter- 

 action, the calculations again show that only a small proportion (1.5% of the 

 collisions at sperm density 10'/ ml) of the collisions are successful in fertilizing 

 the egg. Since the probability of a successful sperm-egg collision appears to 

 be low, the possibility of a rather slow block to polyspermy — of the order of 

 seconds rather than fractions of a second — must be entertained. The calculations 

 suggesting this are based on a number of assumptions, including the assumption 

 that a sperm suspension may be treated as a collection of gas molecules. Roth- 

 schild and Swann justify their assumptions insofar as available data permits. 

 Nevertheless, confirmation of the rate of propagation of the block to poly- 

 spermy by an independent method would seem to be desirable. Direct measure- 

 ments of the time required for the completion of the block to polyspermy 

 were accordingly made. The experimental procedure was designed to fit the 

 following premise: If all the eggs in a batch are fertilized at time / = o and 

 the time required to complete the block to polyspermy is 5 seconds, then there 

 will never be more polyspermic eggs than at 5 seconds. Furthermore, if the 

 sperm and eggs are separated (killing the sperm but not the eggs with hypotonic 

 sea water) at interv^als between / = o and / — 5 seconds, an increasing pro- 

 portion of polyspermic eggs should be found as one approaches / = 5 seconds. 

 Conversely, the time required to just produce the complete block to polyspermy 

 will be the time required to just j)roduce the maximum number of polyspermic 

 eggs. Using this method, Rothschild and Swann (92) obtained values for the 

 conduction time of the complete block to polyspermy ranging from 17 to 94 

 seconds in a series of 14 experiments. The mean value for all the experiments 

 was 63 seconds. 



This astonishing value for the conduction time of the block is far in excess of 

 the time (20 seconds) required for propagation of the visible dark field response 

 of the egg cortex. Evidently, then, the visible dark field color change is not 

 the block to polyspermy. In fact, the 63-second value compares favorably 

 with the time required for initiation of fertilization membrane formation. 



The 63-second value appears unrealistic from a subjective point of view. 

 Therefore, the absence of polyspermy at reasonably high sperm concentrations 



