REPRODUCTIVE ORGANS IN THE MALE. 277 



tunica albuginea. The three tunics just mentioned 

 form the wall or capsule of each testicle, and are so 

 closely associated that it is difficult to distinguish 

 one from the other. 



Structure. The testis is a compound tubular gland 

 divided into three hundred to four hundred lobules. 

 Each lobule is conical in shape with the apex directed 

 toward the mediastinum and the base toward the 

 surface of the organ. The lobules differ in size ac- 

 cording to their 

 position. Each 

 lobule represents 

 several coiled tu- 

 bules which, when 

 unraveled, aver- 

 age two feet in 

 length. There 

 are at least six 

 hundred to eight 

 hundred of these 

 tubules t o each 

 testicle. Their 

 walls are lined with stratified epithelium which is in- 

 vested with a thin layer of connective-tissue ele- 

 ments. The epithelium rests upon a basement 

 membrane and may be arranged in at least three 

 irregular groups or layers: i. A layer of cubical 

 cells, with small nuclei, rests upon the basement 

 membrane. The cells of this layer are called sper- 

 matogonia. The columns of Sertoli, or sustentacular 

 cells, also belong to this layer. These columns 

 are elongated columnar cells that extend from the 



Spermatozoa. 



Spermatoblasts. 



Spermatocyles. 

 Spermatogonia. 



Sustentacular cell. 



Fig. 208. Section of convoluted tubules of 

 testicle. 



