I06 FERTILIZATION 



Polyspermy in Mammals. Except in the case of Ornithorhynchus 

 anatinus (Shaw & Nodder), in which it may be that polyspermy 

 is the rule rather than the exception (Gatenby & Hill, 1924), 

 the incidence of polyspermy in mammalian eggs is low, being 

 of the order of 1-2% under normal mating conditions. They 

 should therefore be classified as Type I eggs, in which only one 

 spermatozoon enters. Austin & Braden (1953) published an im- 

 portant paper, which includes a critical review of earlier work, 

 on polyspermy in rats and rabbits. They found that rat and 

 rabbit eggs go through a critical period, depending on the time 

 after ovulation when they come into contact with spermatozoa, as 

 regards their ability to prevent polyspermy. If mating is delayed 

 so that the eggs remain unusually long in the female reproductive 

 tract before fertilization, the incidence of polyspermy goes up 

 quite sharply, which recalls the increases in polyspermy observed 

 in ageing marine invertebrate eggs. These observations, together 

 with the small numbers of polyspermic eggs normally found, con- 

 firm that mammalian eggs belong to the Type I class. Although, 

 for obvious reasons, Austin & Braden were unable to obtain any 

 information about the ultimate fate of polyspermic mammalian 

 eggs, they observed that the supernumerary male pronuclei did 

 not induce the formation of separate spindles near the periphery 

 of the egg, but approached the female pronucleus and contributed 

 to the first cleavage spindle. The spindle, however, was normal 

 and not multipolar. These observations raise a number of questions 

 about the fate of accessory chromosomes, but the answers must 

 await the results of further experiments. A valuable summary of 

 the information available about the occurrence of polyspermy in 

 mammalian eggs has recently been published by Braden et al. 



(1954)- 



Type I Inhibition of Polyspermy. Type I Inhibition, often called 



the Block to Polyspermy, involves a change in the egg surface, 



after the fertilizing spermatozoon has become attached, such that 



further spermatozoa cannot enter the egg. It has been known since 



the nineteenth century that the fertilizing spermatozoon initiates 



some alteration in the egg which prevents re-fertilization, and many 



workers have observed changes in the morphology of the egg 



surface immediately after fertilization, such that a point on the 



surface opposite to where the fertilizing spermatozoon became 



attached is the last to be affected. The wave of granule breakdown 



