ACTIVITIES OF THE MALE REPRODUCTIVE SYSTEM 23 



during the weeks or months which precede the development of the mating 

 instinct. Many species follow this rule. For example, in the bat of the genus 

 Myotis, sperm are produced during the late spring and summer months, while 

 mating or copulation takes place during the fall or possibly early the next 

 spring (Guthrie, '33). In the common newt, Triturus viridescens, spermato- 

 genesis comes to pass during the warm months of the summer, and sperm 

 are discharged from the testis into the reproductive ducts during the late fall 

 and early spring, while copulation is accomplished in the early spring. The 

 testes in this species are quiescent during the cool winter months. In the 

 midwestern ground squirrel, Citelliis tridecemlineatus, spermatogenesis begins 

 in November and is marked during February and March (fig. 11). TTie animal 

 hibernates away the winter months and emerges the first part of April in a 

 breeding condition. Mating occurs in the early spring (Wells, '35). In the 

 garter snake, Thamnophis radix, sperm are produced in the testes in the sum- 

 mer months, stored in the epididymides during the hibernation period in the 

 fall and winter, and used for copulation purposes in the spring (Cieslak, 

 '45). Again, in the Virginia deer, Odocoileus virginianus borealis, studied by 

 Wislocki ('43), active spermatogenesis is realized during the summer and 

 early autumn months, while the mating season or "rut" which results from 

 the driving power of the male sex hormone, is at its peak in October and 

 November (fig. 12). In the fox, Bishop ('42) observed spermatogenesis to 

 begin in the late fall months, while mating is an event of the late winter and 

 early spring. In April and May the seminiferous tubules again assume an 

 inactive state (fig. 13). In the common frog, Rana pipiens, spermatogenesis 

 is present in the summer months and morphogenesis of spermatids into sperm 

 happens in large numbers during September, October, and November. Sperm 

 are stored in the testis over the winter, and the mating instinct is awakened 

 in the early spring (Glass and Rugh, '44). Following the mating season in 

 spring and early summer the testis of the teleost, Fimdulus heteroclitiis, is 

 depleted of sperm until the next winter and spring (Matthews, '38). 



As the seasonal type of testicular activity is present in a large number of 

 vertebrate species, it seems probable that it represents the more primitive or 

 fundamental type of testicular functioning. 



b. Testicular Tissue Concerned with Male Sex-hormone Production 



While one cannot rule out the indirect effects which activities of the semi- 

 niferous tubules may have upon the functioning of the testis as a whole, in- 

 cluding the interstitial tissue, direct experimental evidence and other obser- 

 vations suggest that the interstitial tissue holds the main responsibility for 

 the secretion of the male sex hormone, testosterone, or a substance very 

 closely allied to it. For example, if a testis from an animal possessing a per- 

 manent scrotum is removed from the inguinal bursa and placed within the 

 peritoneal cavity, the seminiferous tubules tend to degenerate, but the inter- 



