98 VERTEBRATE REPRODUCTIVE CYCLES 



First there is the effect of the external environment on 

 nervous activity, second there is the effect of nervous 

 activity on the internal secretion of those hormones 

 which induce gonad maturation, and third there is the 

 effect of the hormones on that further nervous activity 

 which is seen externally as sexual behaviour. Only the 

 broad outlines of these relationships are known, and the 

 whole subject remains a fascinating one for further 

 research. 



