506 ANDREW T. RASMUSSEN 



last of Mtirc'h, or two to three weeks after waking up. The in- 

 terstitial cells do not, however, reach their maximal size till the 

 last of April. There is a distinct decrease in pigmentation. 



4. Regressive spermatogenesis occurs during the last of 

 April and a new cycle begins early in May, while the interstitial 

 cells remain greatly developed for at least two months longer. 



5. By July the testes have returned to their abdominal position, 

 the interstitial cells begin to show^ signs of decrease and by 

 August most of them are reduced to almost nothing more than 

 naked nuclei. The cytoplasmic lipoids have been absorbed or 

 transformed into a new^ crop of pigment w^hich remains as small 

 brown granules in the scanty cytoplasm in the form of a cap 

 covering about one-half of the nucleus. The nucleus is reduced 

 again in size. A number of the interstitial cells do not thus 

 decrease in size but remain large, the lipoids within them having 

 been transformed bodily, so to speak, into a pigmented substance 

 apparently of the same chemical nature as that of the smaller 

 pigment granules in the ordinary interstitial cells. The nucleus 

 of these special pigmented cells frequently is irregular or pyknotic 

 and may be forced to the periphery of the cell. The testis 

 as a whole is reduced to nearly one-eighth of its former size and 

 is darker in color than at any other stage. Spermatogenesis is 

 slowly progressing uninterruptedly. 



6. A review of the literature on the correspondence between 

 interstitial cell activity, spermatogenesis, breeding period and 

 hibernation indicates great variability, with interstitial cell 

 growth more uniformly related to the later and regressive stages 

 of spermatogenesis than to the initial stages, though there seem 

 to be exceptions even to this generalization. 



It affords me great pleasure to acknowledge the guidance received 

 from Dr. B. F. Kingsbury in this research, and the assistance of 

 Mr. R. S. Outsell, in the retouching of the photographs. 



