218 R. R. HUMPHREY 



duced to their minimal size and number. When, on the other 

 hand, spermatogenesis is at its height and free spermatozoa 

 most numerous, the interstitial cells are of maximal size. ]Ma- 

 zetti ('11) confirms these observations of Friedmann with regard 

 to the frogs Rana fusca and Rana viridis. 



Champy ('08, '13) found in another species of frog (Rana 

 esculenta) that the maximal development of the interstitial 

 cell occurs when spermatogenesis is at its lowest, and vice versa. 

 This condition would appear to differ greatly from that reported 

 by Friedmann and jVIazetti in the three species of frogs studied 

 by them. Champy found in the toad (Bufo vulgaris) a condition 

 similar to that in Rana esculenta and quite the opposite, of that 

 described by Friedmann. He also states that in Rana temporaria 

 the interstitial cells develop only after mating and disappear 

 entirely during the progress of spermatogenesis. 



Sigmficance of variation in correlations 



There is, then, in the higher vertebrates, a decided lack of 

 uniformity in the relations between interstitial cell growth and 

 the progress of spermatogenesis, the variation existing even be- 

 tween closely related members of the same class. The explana- 

 tion of this condition, doubtless, is to be sought in the complex 

 relations within the tubules of the testis. In mammals, birds, 

 reptiles, and anurans, the tubuli contorti are lined at all times 

 by a germinal epithelium, from which the germ cells are prolifer- 

 ated. In animals which breed at any time of the year spermatozoa 

 are being matured constantly and shed into the lumen of the 

 tubule. Around the periphery of the tubule, between the lumen 

 and the basal layer of Sertoli cells, spermatogonia persist through 

 all the changes involved in the maturing of the spermatozoa, 

 and before the ripe spermatozoa at any one point have been shed 

 into the lumen, these will already have differentiated, so that 

 spermatocytes will be developing between basal spermatogonia 

 and the lumen. In closely adjacent parts of the tubule will be 

 found other stages, so that spermatogonia, spermatocytes, 

 spermatids, and free spermatozoa, as well as the Sertoli cells, 



