232 NORMAL HISTOLOGY. 



usual number of chromosomes in its nucleus. It is unnecessary to 

 pursue the chain of events through which the spermatid gives rise to 

 the spermatozoon. It may suffice to state that the body of the latter 

 consists of the chromatin of the nucleus ; that the long cilium con- 

 stituting the tail of the spermatozoon is developed from the cyto- 

 plasm ; and that the centrosome of the spermatid is probably con- 

 tained in the middle piece of the spermatozoon (Fig. 214). Even 

 these conclusions are inferences from studies of spermatogenesis 

 in the lower animals, and not from direct studies of that process in 

 man. The latter undoubtedly conforms very closely to the former 

 in all essential details. 



To return to the histology of the test-is : the epithelial cells of the 

 seminiferous tubules rest upon a basement-membrane, which is divis- 



FIG. 215. 



in- 



Basement-membrane from seminiferous tube of the rat. (Ebner.) in, endothelial cells com- 

 posing the external layer ; I, cells, presumably leucocytes, intercalated between the endo- 

 thelial cells. The faint striations upon the endothelial cells represent wrinkles in the 

 homogeneous membrane forming the inner surface of the basement-membrane ; the 

 wrinkling is probably due to a slight shrinkage of the endothelium. 



ible into two layers : first, an internal, extremely delicate, homoge- 

 neous membrane, upon which the epithelial cells rest ; and, second, 

 a layer of endothelial cells (Fig. 215). The latter may bound, at 

 least in places, the lymphatic spaces, which are abundant in the 

 interstitial tissue of the testis. 



Toward the back of the testis the seminiferous tubules unite 



