502 ANDREW T. RASMUSSEN 



gressioii while spermatogenesis goes on uninterruptedly. The in- 

 terstitial cells remain apparently inactive and greatly reduced 

 for many months while spermatogenesis goes on progressively. 

 It is only towards the last phases of spermatogenesis that the 

 interstitial cells show increased activity. This seems to be the 

 most striking correlation, namely, that the interstitial cells follow 

 wth renewed growth somewhat behind the spermatogenic cycle. 

 And in this sense it is conceivable that in the mole and Rena 

 esculenta the interstitial cells are only somewhat more than 

 usually behind. 



The conclusion expressed by Allen ('04), that in the embry- 

 ology of these cells they develop with reference to degenera- 

 tion in the sex glands, is pertinent, although Whitehead ('04) 

 did not confirm this point, claiming that the interstitial cells 

 appear before there are any signs of degeneration. Mazzetti 

 ('11) explains their decrease in the frog in October as being due 

 to the fact that they have accomplished their purpose of absorb- 

 ing the useless testicular elements after intensive spermatogene- 

 sis. Kingsbury ('14) emphasizes this point in connection wdth 

 the homologous cells of the ovary, where they are most numerous 

 about atretic follicles. In old men it appears well established, 

 especially from the extensive work of Spangaro ('01) and the 

 more recent examination of 130 human testes by Kasai ('08), 

 that while there is usually a noticeable increase in the interstitial 

 cells at puberty, the number seems to decrease again during the 

 active sexual life of the individual to again increase with old age, 

 a time when there is more or less atrophy in the tubules. It is 

 certainly difficult to understand why this increased growth should 

 take place late in the spermatogenic cycle or with a cessation of 

 sexuality if this growth has anything to do with the production of 

 an internal secretion that is of importance in the development 

 of the genital tract, secondary sex characters or the sexual 

 instincts. It appears that in frogs (Rana fusca and Rana viridis) 

 spermatogenesis is initiated without any corresponding observ- 

 able change in the interstitial cells and when they are least active. 

 On the other hand in the woodchuck and mole the spermatogenic 

 cycle commences when the interstitial cells are well developed 

 or beginning to decline. 



