HORMONES AND REPRODUCTION 73 



internal reproductive rhythms, and in the second there 

 is the question of the method by which environmental 

 changes are able to influence and control reproductive 

 physiology. 



With reference to the first question, two cycles have 

 been described which can operate independently of 

 environmental control. The first is the oestrous cycle 

 which in animals such as rats and mice continues 

 unchecked (except by pregnancy) throughout the 

 whole year. The average length of cycle is fixed and 

 typical for each species, but so far it has been impossible 

 even to hazard a guess at the way in which this is 

 achieved. It was once thought that the rhythm could be 

 accounted for merely in terms of a see-saw action 

 between the pituitary stimulation of the ovary on the 

 one hand and the ovary inhibition of the pituitary on 

 the other. However, the same relationships between 

 pituitary and gonad exist in the male where there is no 

 vestige of an oestrous cycle, and clearly some other 

 explanation must be sought. 



An exactly similar difficulty must be faced regarding 

 the problem of the internal reproductive rhythm which 

 lies behind the seasonal cycle. While the final control of 

 such cycles is usually exercised by the external environ- 

 ment, it is nevertheless necessary to suggest that some- 

 where inside the body there exists some tissue or organ 

 that experiences a physiological rhythm with a period- 

 icity of approximately one year. 



It is usual to sidestep these difficulties by suggesting 

 that both the oestrous and seasonal rhythms depend on 

 the periodic activity of the anterior lobe of the pituitary 

 or more probably of the hypothalamus (see p. 102). 



II. THE INFLUENCE OF THE ENVIRONMENT 



Finally, although our knowledge of the subject is 

 still only slight, it is necessary to discuss the manner in 

 which external environmental changes are able to affect 



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