THE KEPEODUCTIVE OEGANS. 765 



The spermatozoa are derived from the breaking up of the seminal 

 cells or daughter cells. They must be looked upon as modified cells. 



The occurrence of spermatozoa in the impregnating fluid of nearly 

 all classes of animals, proves that they are essential to the process of 

 impregnation, and their actual contact with the ovum is necessary for 

 its development. 



The seminal fluid is, probably, after the period of puberty secreted 

 constantly, though, except under excitement, very slowly, in the tubules 

 of the testicles. From these it passes along the vasa deferentia into -the 

 vesiculae seminales, whence, if not expelled in emission, it may be dis- 

 charged, as slowly as it enters them, either with the urine, which may 

 remove minute quantities, mingled with the mucus of the bladder and 

 the secretion of the prostate, or from the urethra in the act of defseca- 

 tion. 



To the vesicular seminales a double function may be assigned ; for 

 they both secrete some fluid to be added to that of the testicles, and 

 serve as reservoirs for the seminal fluid. The former is their most con- 

 stant and probably most important office ; for in the horse, bear, guinea- 

 pig, and several other animals, in whom the vesiculae seminales are large 

 and of apparently active functions, they do not communicate with the 

 vasa deferentia, but pour their secretions, separately, though it may be 

 simultaneously, into the urethra. 



There is a complete want of information respecting the nature and 

 purposes of the secretions of the prostate and Cowper's glands. That 

 they contribute to the right composition of the impregnating fluid, is 

 shown both by the position of the glands and by their enlarging with 

 the testicles at the approach of an animal's breeding time. But that- 

 they contribute only a subordinate part is shown by the fact, that, when 

 the testicles are lost, though these other organs be perfect, all procrea- 

 tive power ceases. 



The fluid part of the semen or liquor seminis has not been satisfac- 

 torily analyzed : but Henle says it contains fibrin, because shortly after 

 being discharged, flocculi form in it by spontaneous coagulation, and 

 leave the rest of it thinner and more liquid, so that the filaments move 

 in it more actively. The chief constituents of the semen are said to be a 

 variety of nuclein, which does not contain sulphur ; certain proteids, one 

 of which contains four per cent, of sulphur; lecithin; cholesterin; 

 and extractives. 



