GENERAL CONCLUSIONS 97 



daylength; in the junco the nervous activity may emanate 

 from the proprioceptors in the muscles; in some frogs it 

 may be due to the presence of water on the skin; 

 in the kilHfish it may start vv^ith the sensation of the 

 increasing temperature; in birds it may be reinforced 

 by the nervous excitement of courtship displays, and in 

 the rabbit by that of copulation itself. So it may be 

 concluded that external circumstances exert their effects 

 on the physiology of reproduction through the inter- 

 mediary of the nervous system, and that the type of 

 external stimulus to which the nervous system is attuned 

 may differ widely in different species. 



Once the anterior pituitary gland has been activated 

 it secretes the gonadotropic hormones which stimulate 

 the gonads. These in turn secrete hormones which 

 have the primary function of increasing the rate of 

 cell division throughout the body. By the combined 

 actions of the pituitary and gonad hormones the eggs 

 and spermatozoa grow to maturity, and by the unaided 

 action of the gonad hormones the accessory sexual 

 organs and the secondary sexual characters develop. 



The gonad hormones also have a powerful effect on the 

 nervous system to induce the development of sexual 

 behaviour. Such behaviour is often remarkably com- 

 plex, particularly in birds which may perform extra- 

 ordinary courtship ceremonies. The proper and energetic 

 performance of these ceremonies is necessary to the 

 success of breeding, the effect being obtained through a 

 further nervous stimulation of the anterior pituitary. 

 Thus the sequence of events comes full cycle, the 

 nervous activity induced by the environment being 

 followed by still greater nervous activity associated with 

 courtship activities. In this way the stimulus to the 

 anterior pituitary is maintained and increased until 

 gonad maturation is complete. 



So, with the difficult problem of the inherent 

 physiological rhythm set aside, the story of the sexual 

 cycle of the vertebrates resolves itself into three parts. 



