72 PRELIMINARIES TO FERTILIZATION 



though it may not be as generous as that of the externally ferti- 

 lized egg. Since the mammalian egg has yet to pass through the 

 genital tract it is possible that certain substances reaching the 

 lumen of the tract in the liquor folliculi or in the secretions of 

 the mucosa correspond to those actually carried by externally 

 fertilized eggs. 



The numerous interactions in which the gametes participate 

 with the environment of fertilization and with each other, to- 

 gether with the mechanisms that permit the spermatozoon to 

 reach and enter the egg, may thus be held to be the preliminaries 

 to fertilization. It is the purpose of this review to describe the 

 current trends of research on these events and processes, in the 

 hope of revealing some possibilities for advance in knowledge in 

 the near future. 



Dilution and Activation of Spermatozoa 



At ejaculation the spermatozoa are mingled with fluids from 

 the accessory glands, principally the seminal vesicles and the 

 prostate, and this fluid forms the bulk of the seminal plasma. The 

 degree of dilution of spermatozoa varies considerably between 

 species: in the semen of the ram the spermatozoa constitute 

 about 25 per cent of the total volume, whereas in the semen of 

 the boar this proportion is only about 1 per cent. The total num- 

 ber of spermatozoa, the volume of the ejaculate, and the compo- 

 sition of the accessory fluids also vary widely. Despite the fact 

 that much is known about the nature of the seminal plasma 

 (Mann, 1954), its precise role is far from clear. Certainly it func- 

 tions as a vehicle for the spermatozoa, but it by no means pro- 

 vides an ideal medium for their survival. Moreover, the larger 

 the proportion of seminal plasma in the ejaculate the more unfa- 

 vorable it is as a medium (Gunn, 1936). Spermatozoa can be 

 stored in vitro in artificial diluents that are quite unlike seminal 

 plasma much more successfully than in the natural diluent itself. 

 The fertility of spermatozoa, also, is not dependent upon their 

 admixture with the accessory secretions (Ivanov, 1926; Ham- 

 mond, 1930; Walton, 1930 ) . When epididymal spermatozoa were 



