48 THE TESTIS AND ITS RELATION TO REPRODUCTION 



as a whole must be considered; reproduction is not merely an environmental- 

 pituitary-sex gland relationship. 



F. Characteristics of the Male Reproductive Cycle and Its Relation to 

 Reproductive Conditions in the Female 



As indicated above, reproduction in the male vertebrate is either a con- 

 tinuous process throughout the reproductive life of the individual or it is a 

 discontinuous, periodic affair. In the continuous form of reproduction the 

 activities of the seminiferous tubules and the interstitial or hormone-producing 

 tissues of the testis function side by side in a continuous fashion. In the 

 discontinuous, periodic type of testicular function, the activities of the semi- 

 niferous tubules and of the interstitial tissue do not always coincide. The 

 activities of the seminiferous tubules, resulting in the production of sperm 

 for a particular reproductive cycle, tend to precede, in some species by many 

 months, the activities of the sex-hormone-producing tissue. Evidently, the 

 output of the FSH and LH substances from the pituitary gland are spread 

 out over different periods of the year to harmonize with this activity of the 

 testicular components. 



It will be seen in the next chapter that a continuous breeding faculty is 

 not present in the female comparable to that of the male. All females are 

 discontinuous breeders. In some species, the cycles follow each other with 

 little rest between each cycle unless the female becomes pregnant or "broody." 

 Some have a series of cycles over one part of the year but experience sexual 

 quiescence over the remaining portion of the year. However, in most female 

 vertebrates there is but one reproductive cycle per year. 



In harmony with the above conditions, the continuous variety of testicular 

 function is always associated with the condition in the female where more 

 than one reproductive cycle occurs per year. Continuous reproductive con- 

 ditions in the male, therefore, are adapted to serve one female two or more 

 times per year or several different females at intervals through the year. 

 Furthermore, the complicated, highly glandular, greatly extended type of 

 male-reproductive-duct system is adapted to conditions of ( 1 ) continuous 

 breeding, or (2) service to more than one female during one breeding season 

 of the year, whereas the simple type of reproductive duct is adapted to the 

 type of service where all or most of the genital products are discharged during 

 one brief period. In other words, the entire male reproductive system and repro- 

 ductive habits are adapted to the behavior of female reproductive activities. 



