44 THE TESTIS AND ITS RELATION TO REPRODUCTION 



the above conditions are contained witiiin tiie body of the organism, and as 

 such represent organismal conditions. 



The following question naturally arises: Do factors or conditions external 

 to the body impinge themselves in such a way as to control pituitary and 

 gonadal function? 



a. Light as a Factor 



Aside from the supply of nutritive substances or the collision of the many 

 nervous stimuli with the individual which may arouse or depress the sexual 

 activities, two of the most important obvious external factors are temperature 

 and light. Research on the reproductive behavior of many animal species, 

 during the past twenty years, has shown that both of these factors have great 

 significance on the reproductive activities of many vertebrate species. Bisson- 

 nette ('30, '32, '35, a and b) has accumulated evidence which demonstrates 

 that light is a potent factor in controlling the reproductive behavior of the 

 European starling (Sturnus vulgaris) and also of the ferret (Putorius vulgaris). 

 In the starling, for example, the evidence shows that green wave lengths of 

 the spectrum inhibit testicular activity, while red rays and white light arouse 

 the reproductive function (fig. 23). The addition of electric lighting to each 

 day's duration produced a total testis size in midwinter which surpassed the 

 normal condition in the spring. In the ferret artificially increased day length 

 beginning at the first part of October brings the testis to maximum size and 

 activity coupled with a normal mating impulse as early as November and 

 December (fig. 24). Under normal conditions the male ferret is able to breed 

 only during February and early March. 



These findings relative to the influence of light on the reproductive perio- 

 dicity of animals confirm a fact which has been known for a long time, 

 namely, that seasonal breeders brought from the northern hemisphere to the 

 southern hemisphere reverse their breeding season. For example, ferrets which 

 normally breed from spring to summer in the northern hemisphere shift their 

 breeding habits to the September-February period when moved to the southern 

 hemisphere. Inasmuch as the hypophysis is instrumental in bringing about 

 secretion of the gonadotrophic hormones responsible for the testicular activity, 

 it is highly probable that light coming through the eyes (see Hill and Parkes, 

 '33) influences the nervous system in some way arousing the hypophysis and 

 stimulating it to secrete these substances in greater quantity. However, one 

 must keep in mind the caution given by Bissonnette, that light is not the only 

 factor conditioning the sexual cycles of ferrets and starlings. 



While numerous animals, such as the migratory birds, ferret, mare, many 

 fish, frogs, etc., normally are brought into a breeding condition during the 

 period of light ascendency, a large number of animals experience a sexual 

 resurgence only during the time of year when the light of day is regressing 

 in span. This condition is found in some sheep, goats, buflfalo in nature, 



