THE REPRODUCTIVE ORGANS 67 



commencement, but gradually widens as it 

 approaches the cloaca. 



Tlie surface of the testis is invested with 

 a firm fibrous covering continuous with a 

 supporting framework in the interior. The 

 essential part of the organ consists of tortuous 

 seminiferous tubules (Fig. 36) containing several 

 layers of epithelial cells from which are derived 

 the male germ -cells or spermatozoa. Lining 

 each tubule is a continuous layer of mother- 

 sperm cells, or spermatogonia, from which are 

 derived all the other cells contained within the 

 tubule. Within the spermatogonia, i.e, nearer 

 the lumen of the tubule, are somewhat larger 

 cells, the spermatocytes, which give origin to 

 spermatids, the forerunners of the spermatozoa 

 (Fig. 37). 



The development of spermatozoa from 

 spermatogonia is an interesting and important 

 process. The male germ-cell in its genesis 

 passes through stages comparable to those 

 experienced by the female germ-cell ; that is, 

 spermatogenesis, of which a very short account 

 may be fittingly given here, is comparable to 

 oogenesis, of which an equally brief account 

 will be given later. The phases through which 

 both kinds of germ-cell pass are divisible into 



