46 THE TESTIS AND ITS RELATION TO REPRODUCTION 



deer, some fish, etc. Bissonnette ('41 ) working with goats found that: "In- 

 creasing daily light periods from January 25 to April 5 — followed by diminish- 

 ing periods until July 5, while temperatures remained normal for the seasons, 

 with four Toggenburg female goats and one male Toggenburg and one Nubian 

 female — led to cessation of breeding cycles in February instead of March, 

 followed by initiation of breeding cycles in May and June instead of Sep- 

 tember." In the ewe, Yeates ('47) also found that a change from increasing 

 daylight to decreasing length of day induced reproductive activity. In a similar 

 manner, Hoover and Hubbard ('37) were able to modify the sexual cycle 

 in a variety of brook trout which normally breeds in December to a breeding 

 season in August. 



b. Temperature Influences 



In the case of the animals mentioned above, temperature does not appear 

 to be a major factor in inducing reproductive activity. However, in many 

 animals temperature is vitally influential in this respect. For example, in the 

 thirteen-lined spermophile (ground squirrel) Wells ('35) observed that breed- 

 ing males kept at 40' F. continued in a breeding condition throughout the 

 year. Under normal conditions this rodent hibernates during the winter months 

 and comes forth in the spring ready to breed; sperm proliferation and general 

 reproductive development take place during the period of hibernation. As 

 the temperature rises during the spring and summer, testicular atrophy ensues, 

 followed by a period of spermatogenesis and reproductive activity when the 

 lowered temperatures of autumn and winter come again. Light, seemingly, is 

 not a factor in this sexual cycle. Another instance of temperature control 

 occurs in the sexual phase of the common red newt, Triturus viridescens. Here 

 it is the rising temperature of the summer which acts as the inducing agent, 

 and sperm thus produced are discharged into the accessory ducts during the 

 fall and winter to be used when copulation occurs in early spring. However, 

 if this species is kept at a relatively low temperature of 8 to 12° C. during 

 the summer months, spermatogenesis is inhibited and the testis regresses. In 

 the stickleback, Gasterosteus aculeatus, as reported by Craig-Bennett ('31), 

 spermatogenesis occurs during July to early September and appears to be 

 conditioned by a rising temperature, whereas the interstitial tissue and the 

 appearance of secondary sexual features reach their greatest development 

 under increased light conditions and slowly rising temperatures (fig. 15). 

 Bissonnette, in his work on ferrets, also observed a difference in the behavior 

 of these two testicular components; the interstitial tissue responds to large 

 increases of daily light periods, whereas the seminiferous tubules are stimu- 

 lated by small, gradually increasing periods of light. 



The above examples emphasize the importance of a single environmental 

 factor on the pituitary-gonadal relationship. However, in the hedgehog, 

 Allanson and Deansley ('34) emphasize temperature, lighting, and hormone 



