GROWTH AND REPRODUCTION 



765 



of the seminal fluid; (3) the intromittent organ, or penis, by means of 

 which the semen is introduced into the female during the act of sexual 

 intercourse, or coitus. 



The Testes. The testes consist essentially of two sets of cells 

 (1) the interstitial cells, which play a part in the acquisition of second- 

 ary sexual characteristics; (2) the germinal cells, from which the 

 spermatozoa are developed, and passed by a long system of ducts 

 to the exterior. The testis is enclosed in a thick capsule, known as 

 the tunica albuginea. From this capsule septa pass into the testis, 

 dividing it into a number of compartments. In the compartments 

 are long convoluted seminal tubules, lined by the germinal epithelium. 

 In the different layers of epithelium the various stages of the " matura- 

 tion of the spermatogonia," spermatogenesis, or may be seen. Most 

 externally, lying upon the basement membrane, are the spermatogonia, 

 supported by elongated " nurse cells," or the " cells of Sertoli." Next 

 comes two layers of spermatocytes large cells with marked karyo- 

 kinetic niclei and then the layer of spermatids small cells with a 

 well-marked rouni nucleus (Fig. 471). 



tail - 



riddle piece \ 

 head g) 



spermatozoon 



spermatocyte II 

 spermatocyte I 

 spermatogone 



wall of tubule 



A, Diagram of a spermatozoon; B, diagram showing the origin of spermatozoa from 

 the living cells (spermatogonia) of the tubules of the testicle. (Kdth.) 



The spermatids also exhibit various stages of transformation to 

 spermatozoa. In this process the nucleus becomes elongated to form 

 the head of the cell, the main mass of cytoplasm goes to form the 

 middle piece, while a filament of cytoplasm grows out to form the 

 whip-like tail (see Fig. 471). When fully mature, the spermatozoa 

 become detached, but connect themselves for a time with the free end 

 of the Sertoli cells. 



The seminal tubules are supported by a number of fine connective- 

 tissue fibres, in which run the bloodvessels and lymphatics, and in 

 which are also situated the interstitial cells which play so important a 

 part in the acquisition of the secondary maje characteristics (see p. 505). 

 The convoluted seminal tubules of each compartment pass to join with 

 a few straight tubules (the tubuli recti), to form a network known as 

 the rete testis. From this emerge the vasa efferentia (twelve to fifteen 



