C. R. AUSTIN AND M. W. H. BISHOP 85 



seems to occur in ferns, mosses, and seaweeds (Rothschild, 

 1951a), but so far no unequivocal evidence of attraction between 

 animal gametes has been brought forward; this was pointed out 

 many years ago by BuUer ( 1903 ) and it is apparently still true 

 today (Tyler, 1955). Certain other features may favor the chances 

 of fertilization: the large cumulus mass about many mammalian 

 eggs may help by providing a larger target for the spermatozoa, 

 and the radially arranged cumulus cells, particularly in the near 

 vicinity of the egg, may aid by orientating the spermatozoon 

 toward the egg (Austin and Braden, 1952). 



The chances of fertilization are, however, chiefly related to the 

 concentration of spermatozoa about the eggs, and this will de- 

 pend both on the total number of spermatozoa present and on the 

 space through which these are distributed, that is, on the size of 

 the site of fertilization. Many more spermatozoa were found at 

 the site of fertilization in the sheep and rabbit than in the rat and 

 mouse (as noted earlier), but the site of fertilization in the for- 

 mer two animals is much larger than in the latter two. The egg 

 itself, the ultimate target for the spermatozoon, also differs in 

 size in these species: sheep and rabbit eggs have approximately 

 four times the surface area of rat and mouse eggs. When all these 

 variables are taken into account, the collision frequency between 

 eggs and spermatozoa is found to be of the same order in all four 

 species (Braden and Austin, 1954a). These considerations relate 

 particularly to the chances of penetration of individual eggs; data 

 on the chances of penetration of the eggs as a group, in any 

 one animal or Fallopian tube, have been reported by Chang 

 ( 1946b,c ) . He found that, when superovulation was induced in 

 rabbits by hormone administration, a higher proportion of eggs 

 was fertilized than in untreated rabbits inseminated with the 

 same numbers of spermatozoa. Evidently either the increased 

 numbers of eggs or the effect of hormone treatment in some way 

 improved the chances of successful fertilization and more than 

 compensated for the need for the penetration of an abnormally 

 large number of eggs. Estrogen administration in mice has also 

 been found to increase the incidence of fertilization, presumably 

 by stimulating uterine and tubal movements and thus increasing 



