240 R. R. HUMPHREY 



collecting duct toward the periphery of the testis (figs. 4 and 

 5). During such growth in Desmognathus there are no intersti- 

 tial cells present, since in this urodele as well as in Salamandra 

 atra and Diemyctylus lobule regeneration is delayed until after 

 complete disappearance of interstitial cells from the region. The 

 lobule development clearly could not, therefore, in these urodeles 

 affect directly the interstitial cells or be affected thereby. Though 

 in Necturus the interstitial cells are still present, whenever they 

 chance to be included between the outer ends of adjacent grow- 

 ing lobules they either degenerate or revert to the stromal cell 

 type from which they arose, never persisting long as interstitial 

 cells between lobules of spermatogonia (figs. 15 and 16). This 

 condition is quite the contrary of what one would expect if they 

 were to serve the ' trophic' function ascribed to them by some in- 

 vestigators (e.g., Plato, '97). 



The modification of stromal into interstitial cells, therefore, 

 is clearly a temporary response to conditions within the lobule; 

 when such favoring conditions no longer prevail, the original 

 cell type may be resumed or the cells may degenerate, the prox- 

 imity of the regenerated lobules in Necturus apparently hastening 

 these processes. 



A peculiar aspect of the modification of stromal into interstitial 

 cells is to be noted in the anterior end of the testis, in which 

 spermatozoa may remain in some animals as late as April. When 

 the lobules are finally emptied or the spermatozoa within them 

 undergo degeneration, the stromal cells surrounding each lobule 

 begin their usual changes; and, in the case of the lobules earlier 

 emptied, may form a sheath of enlarged cells one cell layer in 

 thickness. The tendency, however, is for cells surrounding these 

 late-emptied lobules to undergo comparatively little change, the 

 majority of them never progressing beyond the earlier stages of 

 modification. In May, instead of a complete ring of interstitial 

 cells around cephalic lobules, one finds perhaps two or three 

 enlarged cells laden with fat droplets, as in figure 13, the other 

 surrounding stromal cells have the flattened form and few or no 

 fat droplets. That mitoses in this region have been suppressed 

 also may be inferred from the comparatively small number of 



