INTERNAL FACTORS AND TESTICULAR FUNCTION 47 



injections as factors modifying tiie sexual cycles, while Baker and Ransom 

 ('32, '33, a and b) show that light, food, temperature, and locality affect 

 the sexual cycles and breeding habits of the field mouse. In some vertebrates, 

 therefore, a single factor may be the dominant one, whereas in others, numer- 

 ous factors control the action of the pituitary and reproductive system. 



E. Internal Factors Which May Control Seasonal and Continuous Types 



of Testicular Function 



In endeavoring to explain the differences in response to external environ- 

 mental factors on the part of seasonal and continuous breeders, one must 

 keep in mind the following possibilities: 



(1) The anterior lobe of the hypophysis in some forms (e.g., ferret) 

 cannot be maintained in a secretory condition after it has reached its 

 climax; that is, it apparently becomes insensitive to the light factor. As a 

 result, regression of the pituitary and testis occurs (Bissonnette, '35b). 



(2) In the starling, the anterior hypophysis may be maintained by the 

 lighting, but the testis itself- does not respond to the presence of the 

 hypophyseal hormones in the blood (Bissonnette, '35b). The possi- 

 bility in this instance may be that testicular function wanes because 

 the body rapidly eliminates the hormone in some way (see Bachman, 

 Collip, and Selye, '34). 



(3) Consideration also must be given to the suggestion that the activities 

 of the sex gland by the secretion of the sex hormone may suppress 

 anterior lobe activity (Moore and Price, '32). 



We may consider two further possibilities relative to continuous testicular 

 function: 



(4) If the "brake actions" mentioned above are not present or present 

 only in a slight degree, a degree not sufficient to interrupt the activities 

 of the anterior lobe or of the sex gland, a more or less continuous 

 function of the testis may be maintained. 



(5) When several or many environmental factors are concerned in pro- 

 ducing testicular activity, a slight altering of one factor, such as light, 

 may prove insufficient to interrupt the pituitary-germ-gland relation- 

 ship, and a continuous breeding state is effected in spite of seasonal 

 changes. 



Underlying the above possibilities which may control testicular function is 

 the inherent tendency or hereditary constitution of the animal. In the final 

 analysis, it is this constitution which responds to environmental stimuli, and 

 moreover, controls the entire metabolism of the body. In other words, the 

 above-mentioned possibilities tend to oversimplify the problem. The organism 



