88 THE MAMMALIAN EGG 



findings have been made on sperm penetration in Hydroides (A. L. 

 Colwin and L. H. Colwin, personal communication, i960). 



The properties of the sperm head and vitelline membrane that 

 permit attachment can evidently be abolished — many spermatozoa 

 treated with hyaluronidase inhibitor seem unable to stick on the 

 vitelline surface (Parkes, Rogers and Spensley, 1954) and eggs sub- 

 jected to heat treatment often appear to have an impermeable 

 vitcllus (Austin and Braden, 1956). There is evidence too that these 

 properties of sperm head and vitelline membrane are subject to 

 genetic influence; Krzanowska (i960) reports that the low fertility 

 of an inbred strain of mice (E strain) could be attributed to a low 

 fertilization rate, and that a remarkably high proportion of the un- 

 fertilized eggs (varying from 13*1 to 18-7 per cent) contained 

 spermatozoa in the perivitelline space. The eggs were not activated 

 either, which certainly implies that no attachment to the vitelline 

 surface had occurred. The proportion of such eggs was greatly 

 reduced by outcrossing in either direction. 



Attachment of the spermatozoon to the vitelline membrane is 

 generally effected only by the first one to make contact with it, and 

 subsequent spermatozoa are thus unable to pass into the vitellus and 

 take part in fertilization. The change in reactivity of the vitelline 

 surface reflects the operation of the block to polyspermy, a defence 

 mechanism protecting the egg against the occurrence of polyandry 

 (p. 41). The efficiency of the block to polyspermy has been found 

 to vary in different stocks and strains of rats and mice (Table 3). In 

 the sea-urchin egg, the block to polyspermy is considered to be a 

 change propagated over the egg cortex in two phases: a fast partial 

 block affects the whole surface in one or two seconds and a complete 

 block is established in about 60 sec (Rothschild, 1954, 1956; Roths- 

 child and Swann, 1949, 195 1, 1952). Whether the mammalian block 

 to polyspermy is biphasic and how long it takes to pass over the 

 vitelline surface are, as yet, unanswered questions. Some similarities, 

 however, have been demonstrated — in both groups of animals the 

 block loses efficiency, presumably by slowing down, as the egg 

 becomes stale or ages, and this change is hastened by heat treatment. 

 The aging effect in mammalian eggs is shown by the greater fre- 

 quency with which polyspermy is encountered in animals that have 

 copulated or been inseminated fiear the end of oestrus (p. 43), and 

 the effect of the local application of heat or of the induction of 

 hyperthermia is summarized in Table 3. 



