THE REPRODUCTIVE ORGANS 69 



By a process of metamorphosis without further 

 division the spermatids become spermatozoa. 



The fully formed spermatozoon is a small 

 elongated cell to which a long and actively 

 motile flagellum is appended. A spermatozoon 

 consists of three parts : a head, an intermediate 

 part or neck, and a tail. The head contains 

 the nucleus derived from the spermatid ; 

 while the neck contains the centrosomes. The 

 tail has been described as consisting of three 

 portions. A pars conjunctionis unites the tail to 

 the neck ; a pars principalis constitutes the main 

 length of the tail ; and a pars terminalis con- 

 sists solely of an axial filament which traverses 

 the entire tail and is surrounded by a proto- 

 plasmic sheath in the other two parts. 



Here and there, in the connective tissue 

 between the seminiferous tubules, are clusters 

 of polyhedral cells, with rounded nuclei, appar- 

 ently differing in character from the neigh- 

 bouring cells. These are generally referred 

 to as interstitial cells, and by some are held 

 to be responsible for the production of the 

 secondary sexual characters. In the active 

 testis fat globules are demonstrable in the 

 tissue between the seminiferous tubules, and 

 even in the tubules themselves. 



