SPERM DUCTS 239 



gonia may he ai)])ar(.'nt. If one ero]) of spermatids is {•()m])leting 

 metamorphosis into sperm, then mitotic aeti\ ity may ah-eady have 

 })eo;un in the spermatoii;onia initiating another ^rou]) of s])ermati{ls. 

 In many mannnals it a])])ears tiiat spermatogenie activity passes 

 along the seminiferous tuhiile in waves from their (Hstal ends toward 

 the efferent ducts. Spermatogenesis has l)een intensively stiuHed 

 in the rat, where it has been estimated that it takes about twenty 

 days for a spermatogonia! cell to develop into mature sperm. 



This spermatogenic activity is limited to animals that have 

 reached the age of maturity. Breeding seasons usually occur during 

 this period of activity. 



Interstitial Cells. — Between the adjacent seminiferous tubules is 

 a loose fibroelastic connective tissue similar to that forming the 

 wall of the tubules. In this tissue region are the blood and 

 lymph vessels, nerves, and so-called interstitial cells. (Fig. 149.) 

 These cells appear to have about the same size as the granular 

 leukocytes and show \ariation in shape. If they are connective- 

 tissue cells they a])])ear to be a special variety and their acti^•ity 

 is thought to be endocrine in nature. The spherical nucleus is 

 relatively large, with coarse chromatin granules and one or two 

 nucleoli; the cytoplasm shows variation in the degree of granulation 

 of a fatty nature. Transitional forms indicate the origin of these 

 cells from fibroblasts and their return to a fibroblast type imder 

 conditions of inflammation. The true nature of these cells still 

 remains imdetermined. Endocrine secretion on the part of the 

 testis is evidenced by the effects following its removal and trans- 

 plantation. For exami)le, if the testes are remo^•ed from an innna- 

 ture animal, the secondary sexual characters of males do not de\elop; 

 if this removal takes place after maturity there is a loss of sex emo- 

 tions and reproductive activities, and the body accumulates con- 

 siderable adipose tissue. The transplantation of a testis into the 

 body of a female causes secondary sexual characters of the male 

 to make their appearance in the absence of the ovaries. A number 

 of such ex])eriments point to an endocrine activity on the part of 

 the testis, luit do not indicate the tissue directly res])onsible. 



SPERM DUCTS. 



In studying the urinary system attention was called to the fact 

 that in the male parts of the mesonephric tulniles became asso- 

 ciated with the testis as eft'erent ducts for the sperm. At the same 

 time the seminiferous tubules are developing in the testis, cells of 



