236 THE MALE REPRODUCTIVE SYSTEM 



in the walls of the mature testis tubules and are much more numer- 

 ous. (Fig. 147.) The process of sperm cell formation from the 

 spermatogonial cells is called spermatogenesis. Before maturity of 

 the animal there is a series of abortive efforts to carry through 

 development of sperm to completion, until finally the process is 

 well established and fully differentiated and functional sperm are 

 regularly produced. 



Testes, like ovaries, are not always active but show seasonal 

 periods whose duration and occurrence varies with the species. 

 Both testes and the sperm ducts show variations, depending on 

 whether they are studied during or between breeding seasons. 

 During the inactive stage following or preceding a breeding season, 

 most of the spermatogonial cells have degenerated, the lumens of 

 the tubules are lost, interstitial connective tissue between the adja- 

 cent tubules increases in amount, and cords or spheres of large rest- 

 ing spermatogenic and Sertoli cells are prominent. The conditions 

 at these times resemble those of the immature or developing testis 

 before spermatogenesis has begun. To study the acti\'e condition 

 it is necessary to secure the testis of an animal during the breeding 

 season. 



The vertebrate testis is usually a compound tubular gland, and 

 the seminiferous tubules, where spermatozoa are formed, represent 

 the secretory end-pieces. They are connected with a duct system 

 by which the sperm pass to the outside. 



SPERMATOGENESIS. 



In general, spermatogenesis shows little variation among verte- 

 brates, the series of divisions of cells and differentiation of the sperm 

 presenting similar stages. The wall of each seminiferous tubule 

 consists externally of circularly and longitudinally disposed col- 

 lagenous and elastic fibers and connective-tissue cells. Between 

 adjacent tubules there are regions filled wath interstitial tissue 

 composed of loose fibroelastic connective tissue and certain special 

 elements, called interstitial cells, thought to be secretory. Within 

 the connective-tissue sheath of each tubule is a basement membrane 

 on which rests the highly specialized stratified germinal ei)itlielium 

 composed of Sertoli cells and spermatogonial cells. (Fig. 149.) 



Sertoli Cells.— These are long columnar-like cells with broad bases 

 resting on the basement membrane, with the elongated jjortions of 

 the cells extending like si)okes of a wheel part way toward the center 



