24 The Biochemistry of Semen 



The presence of the other kind of reducing substances can be 

 detected in protein-free filtrates from semen and seminal vesicle 

 fluid, by heating with sugar reagents, such as for example, cupric 

 hydroxide. In this category belongs fructose, the physiological sugar 

 of semen. 



Huggins and Johnson (1933) were the first to observe that the 

 reducing sugar of human semen is derived from the secretion of the 

 seminal vesicles but is absent in the prostate. Similar findings were 

 made with the bull (Bernstein, 1937), boar (McKenzie, Miller and 

 Bauguess, 1938) and ram (Moore and Mayer, 1941). The identifi- 

 cation of the seminal sugar as fructose (Mann, 1946a, b, c) opened 

 the way for detailed studies of the fructose-generating capacity of 

 the accessory tissues (Fig. 4). It was shown that in several species 

 fructose is secreted either by the seminal vesicles or by functionally 

 related organs (Mann, 1946c; 1947; l94Sa, b). This made it possible 

 to use the chemical assay of fructose in semen as an indicator of the 

 relative contribution made by the seminal vesicles towards the make- 

 up of the whole semen. It must be pointed out, however, that a 

 certain small amount of fructose is also produced by the ampullar 

 glands and in some species, by certain other accessory organs (see 

 p. 138). 



The physiological function of the seminal plasma 



From time to time doubt is expressed as to whether the individual 

 accessory gland secretions or even the entire seminal plasma, have 

 any essential role to fulfil in the process of reproduction; the more 

 so, since in some anmials such as the guinea-pig or rabbit, it is 

 possible to induce pregnancy by the artificial insemination of epi- 

 didymal spermatozoa. It is however, arguable as to how much sig- 

 nificance may be ascribed to such experiments, and it is certain that 

 the natural mating process could scarcely be expected to function 

 smoothly and efficiently without the provision of seminal plasma 

 as a normal diluent and vehicle for the thick mass of closely packed 

 epididymal spermatozoa; no more could the blood corpuscles act 

 as oxygen carriers in vivo, without the blood plasma. 



Furthermore, the seminal plasma exerts a distinct stimulating 

 effect on sperm motility. In part, this is due simply to the 'dilution 

 effect', a phenomenon which is described fully elsewhere (see p. 73). 



