INTERSTITIAL CELLS OF URODELE TESTIS 219 



will all be present in a limited area of the tubule. Progressive 

 and regressive changes, therefore, must go on side by side, 

 rendering impossible any accurate analysis of conditions which 

 may affect interstitial cell development. 



Animals which have but one rutting period yearly offer more 

 favorable material for study, since in these the various stages of 

 spermatogenesis are more widely distributed as to time. But 

 even in testes of such animals (when we deal with mammals, 

 birds, reptiles, or anura) there is no absolute separation of the 

 progressive from the regressive changes such as would enable 

 one definitely to know with which the interstitial cell growth is 

 correlated. Degenerative changes, for example, occur when 

 the transforming spermatids cast off a part of their cytoplasm, 

 but such changes cannot affect adjacent testicular structures 

 which may not at the same time, be subject to influences due to 

 the progressive changes in the spermatogonia, or be profoundly 

 affected, also, by the varying processes transpiring in the omni- 

 present Sertoli cells. 



If, then, the interstitial cell status at any period is the expres- 

 sion of conditions wdthin the tubules, it must be the expression 

 of several influences acting synchronously rather than of one 

 influence acting without interference. These combinations of 

 influences may be thought of as acting as do the combinations 

 of forces of the physicist: opposing forces tend to neutralize 

 each other, and variations in the relative strengths of forces to 

 change appreciably the direction or extent of the resulting 

 movement. That different animals should differ in details of 

 metabolism is inevitable. A degenerative change, for example, 

 may be slightly delayed in one species and accelerated in another; 

 such a change, then, considered as a force capable of affecting 

 interstitial cell development, is differently applied in the two 

 animals and may lead to a difference in their interstitial cell 

 development — a difference, it is true, subject both to increase 

 and diminution by the action of other influences in the combina- 

 tions. It is, I think, to some such explanation as this that we 

 must turn if we are in any way to harmonize the apparently 

 varied results of previous investigations of cyclic changes. 



