CHAPTER VI 



GENERAL CONCLUSIONS 



THE data summarized in this book relate to the two 

 best known sexual cycles, namely the short oestrous 

 cycle, which is most clearly seen in female mammals^ 

 and the long seasonal cycle, which is common to most 

 vertebrates in all parts of the world. The information 

 now available indicates that these are controlled by two 

 distinct mechanisms. The first is that inherent physio- 

 logical rhythm which is in complete control of the 

 oestrous cycle and in partial control of the seasonal 

 cycle. The second is that physiological rhythm which 

 depends for its development on external environmental 

 changes, and which usually exercises a dominant control 

 over the seasonal cycle. 



As regards the basic mechanism of inherent sexual 

 cycles little or nothing is yet known. Oestrous cycles are 

 genetically fixed, and have a periodicity that is peculiar 

 to each species. It has been suggested that the physio- 

 logical rhythm on which these cycles must be based may 

 be centred in the anterior lobe of the pituitary gland, 

 but it is more probable that it may be centred in some 

 part of the nervous system which dominates the pituitary 

 gland (see p. 102). 



The control which the nervous system may exercise 

 over the activities of the pituitary gland can be clearly 

 shown by an analysis of the seasonal cycle. In an animal 

 such as the ferret which breeds in spring the stimulus of 

 the increasing daylength enters the eyes and passes in the 

 form of nervous activity through the optic nerves and the 

 brain to the pituitary gland. In other species a similar 

 nervous activation of the pituitary may be achieved in 

 other ways. In the sheep it is probable that the eyes are 

 also important but the stimulus is the result of decreasing 



