76 PRELIMINARIES TO FERTILIZATION 



A similar function may reasonably be imputed to the vaginal 

 plugs formed in many rodents, some bats and insectivores, and 

 the chimpanzee. In the opossum a similar plug is apparently 

 formed by coagulation of female secretions by the seminal fluid. 

 The human seminal gel evidently has a different significance for 

 it soon liquefies under the action of a proteolytic enzyme present 

 in the semen (Huggins and Neal, 1942; Huggins and Vail, 1943). 

 Seminal gels may also have the function of stimulating the female 

 tract by mechanical action, and seminal plasma as a whole by 

 viitue of its physiologically active constituents, such as choline 

 and adrenaline (see Mann, 1954). It has been claimed, notably 

 by Russian workers, that spermatozoa penetrate into the wall of 

 the female tract and exert a stimulating influence upon its func- 

 tions (see Kushner, 1954), 



For many species the recorded time interval between coitus 

 and the arrival of spermatozoa in the ampulla of the Fallopian 

 tube is surprisingly short: intervals of 15 minutes or less have 

 been noted in the sheep (Starke, 1949), cow (Van Demark and 

 Moeller, 1951), mouse (Lewis and Wright, 1935), rat (Blandau 

 and Money, 1944), and bitch (Whitney, cited by Evans, 1933). 

 By contrast, spermatozoon transport in the rabbit seems to take 

 about 3 or 4 hours (Heape, 1905; Parker, 1931; Florey and Wal- 

 ton, 1932; Braden, 1953; Adams, 1956), although it may occasion- 

 ally take only one hour (Chang, 1952a). It is possible that the 

 slower rate in the rabbit may be associated with the absence of 

 copious uterine fluid; Warren (1938), however, found that re- 

 moval of the uterine fluid from the rat uterus delayed transport 

 by only a few minutes. It is possible that rabbit spermatozoa 

 depend, much more than those of the other species mentioned, 

 upon their own motility to reach their goal. The rabbit, it would 

 seem, does not require rapid spermatozoon transport because it 

 exhibits copulatory ovulation. The biological advantage of rapid 

 transport to animals showing spontaneous ovulation is presum- 

 ably that it increases the period during which coitus can result 

 in normal fertilization — it would be especially important when 

 coitus occurs close to or soon after ovulation. Experimentally 

 delayed coitus or insemination often results in a large increase 



