I02 VERTEBRATE REPRODUCTIVE CYCLES 



be translated into nervous stimuli which then pass to the 

 brain. In the case of light entering the eyes to stimulate 

 the growth of the gonads, the pathway of the stimulus 

 through the optic nerves to the brain has long been 

 recognised (see p. 75). 



The most important recent development has been the 

 suggestion that, within the brain, the hypothalamus may 

 be a region of special significance in connexion with 

 breeding cycles. The stimulus from the sense organs may 

 pass first to this region and thence via the pituitary 

 gland to the gonads (Donavan and Harris^^^). It is also 

 considered possible that inherent reproductive cycles, 

 which are not influenced by external changes, may be 

 controlled from the hypothalamus. Such cycles may have 

 a periodicity of about a year (see p. 71) or they may be 

 the shorter oestrous rhythms seen in polyoestrous 

 mammals (see p. 67). It is no longer thought probable that 

 rhythms of this sort are situated in the pituitary gland. 



The hypothalamus is well developed in all vertebrates, 

 and when it is damaged or stimulated, changes may 

 occur in temperature regulation, sugar and fat meta- 

 bolism, the rhythm of waking and sleeping, and in 

 sexual activity (Donovan and Harris^®^). In particular, 

 experimental lesions in this region may cause either 

 gonad atrophy or constant oestrous with ovulation. 



It is suggested that efltects of this kind on gonad 

 activity must be mediated through the pituitary gland, 

 and that the message passing from the hypothalamus to 

 the pituitary must be hormonal in nature. This hormone, 

 which has not yet been extracted, evidently passes with 

 the blood through the hypophyseal portal vessels that 

 run lateral to the pituitary stalk. The theory of Thomson 

 and Zuckerman^^*^ that the message may continue to pass 

 in the absence of these vessels seems doubtful, especially 

 in view of the observation of Harris and Jacobsohn^^^ 

 that sexual cycles do not recommence in hypophy- 

 sectomised animals unless a new pituitary is grafted into 

 place beneath the hypothalamus and unless this new 



