The Two Components of Semen 15 



but has instead the much thinner 'tail sheath' or 'cortical helix* 

 which terminates a short distance before the end of the tail, exposing 

 the terminal portion of the axial filament, that is the end-piece. In 

 mammalian spermatozoa, the tail sheath appears as a helicoidally 

 wound cord; when the tail of the spermatozoon is broken, one can 

 see, protruding from the cortical sheath, the brush-like fibrils of the 

 axial filament, and at this point it is also possible to distinguish the 

 helical structure of the tail sheath (Plate II). In fowl spermatozoa on 

 the other hand, there is no evidence of a 'cortical helix', and the axial 

 filament is encased in an amorphous sheath which is easily dis- 

 rupted by distilled water, causing the axial filament to fray into 

 fibrils. In addition to the various fibrous cortical systems, the sperm 

 cell of many species, including man and the higher mammals, is 

 protected externally by a lipid layer or capsule ('manteau lipidique') 

 evident especially around the tail, and composed of a layer of 

 liproprotein. 



SEMINAL PLASMA 



Seminal plasma, the extracellular fluid which provides the medium 

 and vehicle for spermatozoa, originates in the accessory organs of 

 reproduction and varies in composition according to species. In 

 lower animals it may be so scarce that the emitted semen takes the 

 form of a very thick lump of spermatozoa, closely packed together. 

 There is little seminal plasma in bird semen and even among some 

 of the mammals, but on the whole, the higher mammals, including 

 man, produce a relatively dilute semen with a considerable propor- 

 tion of seminal plasma. 



Secretory function of male accessory glands 



The seminal plasma is a composite mixture of fluids secreted by 

 organs which in the higher species comprise the epididymides, the 

 seminal ducts or vasa deferentia, ampullae, prostate, seminal vesicles 

 (or seminal glands), Cowper's glands and certain other glands 

 located in the wall of the urethral canal. Until a little while ago, the 

 secretory function of the male accessory organs remained obscure 

 chiefly owing to lack of information about the chemical nature of 

 the various secretions. More recently, however, several substances 



