78 PRELIMINARIES TO FERTILIZATION 



observations on the passage of radio-opaque fluids. It may also 

 be relevant that hybrids seem to be more common in ungulates 

 than they are among rodents (Gray, 1954). These data suggest 

 that the exclusion of foreign spermatozoa is a mechanical func- 

 tion of the utero-tubal junction, but reactions of an immunologi- 

 cal nature may also be involved. 



Within the environment of fertilization there is an ecological 

 niche of special significance which is more clearly defined in 

 mammals than in species having external fertilization. This is the 

 "site of fertilization" and it is characterized not merely by ana- 

 tomical position but by the fact that here the conditions are j^re- 

 sumably optimal for ensuring the penetration of the spermato- 

 zoon into the egg — they may not necessarily be the best for 

 maintaining the viability of the gametes or of the zygote. In most 

 mammals the site of fertilization is the ampulla of the Fallopian 

 tube; in some species spermatozoon penetration may occur in the 

 peri-ovarian sac or bursa which can thus be regarded as an ex- 

 tension of the Fallopian tube, physiologically as well as anatomi- 

 cally (see Braden and Austin, 1954a). A notable exception to the 

 general rule is provided by the tenrecs, in which penetration ap- 

 parently occurs while the egg is still in the ovarian follicle ( Blunt- 

 schli, 1938; Strauss, 1938, 1950). That fertilization within the 

 follicle may occasionally take place in other species also is shown 

 by the fact that ovarian pregnancies have long been known in 

 man (Mahfouz, 1949). The number of spermatozoa that have 

 been observed at the site of fertilization is surprisingly small, 

 especially when compared with the many millions deposited at 

 coitus: rat, mean number of 43 spermatozoa per Fallopian tube 

 (Austin, 1948b), mean of 12 spermatozoa (Blandau and Odor, 

 1949); mouse, mean of 17 spermatozoa (Braden and Austin, 

 1954a); rabbit, less than 1000 spermatozoa (Austin, 1948b; 

 Chang, 1951b), mean of 500 spermatozoa (Braden, 1953); sheep, 

 mean of 340 spermatozoa (Braden and Austin, 1954a). 



In view of the known influence of ovarian function upon the 

 spontaneous muscular activity of the uterus (Reynolds, 1949; 

 Wislocki and Guttmacher, 1924; Cupps and Asdell, 1944; Evans 

 and Miller, 1936) and on the nature of secretions of the repro- 



