86 PRELIMINARIES TO FERTILIZATION 



the number of spermatozoa available for fertilization ( Austin and 

 Bruce, 1956). In rats a highly significant association was shown 

 to exist between the number of spermatozoa at the site of fer- 

 tilization and the incidence of extra spermatozoa within eggs 

 (Braden and Austin, 1954a). The data indicate, therefore, that 

 the collision frequency between eggs and spermatozoa is condi- 

 tioned by the mechanical and physical attributes of the female 

 tract in such a way as normally to provide all eggs with good 

 chances of fertilization without too great a risk of penetration by 

 excessive numbers of spermatozoa. 



However, the eggs themselves also have mechanisms for pro- 

 tection against penetration by extra spermatozoa, namely the 

 zona reaction and the vitelline block to polyspermy, and these 

 devices will be discussed later. For the present it is sufficient to 

 note that in general the entry of extra spermatozoa into the eggs 

 of marsupial and placental mammals must be considered abnor- 

 mal although probably not disadvantageous to the embryo, pro- 

 vided they remain within the perivitelline space. Kushner ( 1954), 

 indeed, reports that Russian workers have observed improved 

 fertility in livestock following insemination with the mixed se- 

 men of several sires. The effect was believed to be due in part to 

 the entry into the egg of several spermatozoa originating from 

 more than one male. The rabbit egg can certainly tolerate the 

 presence of numerous spermatozoa in the perivitelline space ( 20, 

 Van Beneden, 1875; up to 50, Hensen, 1876; a mean of 37, Mori- 

 card and Bossu, 1949; a mean of 17, Chang, 1951b; a mean of 72, 

 Braden, Austin, and David, 1954; up to 20, Adams, 1955). Even 

 in the rabbit egg, however, there is a limit to the invasion: at 

 about 6 hours after ovulation a layer of mucoprotein begins to 

 accumulate on the surface of the zona ( Braden, 1952, 1953 ) , and 

 this layer is impenetrable to spermatozoa (Pincus, 1930; Ham- 

 mond, 1934). Occasionally, more than one spermatozoon enters 

 the vitellus of the mammalian eggs and more than one male pro- 

 nucleus is formed. This constitutes the state of polyspermy which 

 is accepted as being pathological in eutherian mammals, although 

 it is apparently normal in several other species, notably among 

 the insects, elasmobranchs, amphibians, reptiles, birds, and pos- 



