HORMONES AND REPRODUCTION 77 



pituitary will cause ovarian development and ovulation 

 in birds, and that sexual posturing or even the mere 

 association of two individuals will initiate nest-building 

 and ovulation. There is a presumption, therefore, that 

 sexual posturing produces stimuli which act upon the 

 anterior pituitary through the hypothalamus, and so 

 effects the necessary synchronisation between the 

 sexual processes of the male and female birds. Herein 

 then, in all probability, lies the biological or race-survival 

 value of sexual display and of the adornment which in 

 many species is taken advantage of to render the display 

 more effective. Those birds which have brighter colours, 

 more elaborate ornamentation, and a greater power of 

 display must be supposed to possess a superior capacity 

 for effecting by pituitary stimulation a close degree of 

 physiological adjustment between the two sexes so as to 

 bring about ovulation and the related processes at the 

 most appropriate times.' 



Finally it remains to consider the effect of copulation 

 on ovulation since this too demonstrates the important 

 influence of the nervous system on the anterior pituitary. 

 While this nervous mechanism is probably present in 

 most mammals, it is best seen in such a species as the 

 rabbit. Here it is so highly developed that ovulation 

 always occurs automatically after copulation. The act 

 of mating initiates a train of events, the first step in 

 which is the passage of a wave of nervous excitation to 

 the brain and so to the pituitary. The anterior lobe then 

 begins to discharge its stored gonadotropic hormone 

 which induces ovulation within 24 hours. If the con- 

 nexion between the pituitary and the brain is cut 

 ovulation does not occur,*^' ^^ and, of course, the same is 

 true if the pituitary is removed altogether. Conversely, 

 ovulation can be induced without copulation if gonado- 

 tropic hormone is injected. 



This effect can also be obtained experimentally by 

 inserting a glass rod into the vagina, or even by the mere 

 sight of a male. It is amusing to recall that when the 



