^5 VERTEBRATE REPRODUCTIVE CYCLES 



has been confirmed, and it is evident that the pathway of 

 stimulation Hes along the optic nerve. In the higher 

 vertebrates, with bodies covered with feathers or fur, 

 there is little possibility of other light receptors being 

 involved, and Benoit's observations on this point must 

 be regarded with scepticism. However, among the lower 

 vertebrates other light sensitive organs may be found, 

 and indeed Scharrer^'^ j^gg shown that blinded minnows 

 are still capable of appreciating changes in light intensity. 



More recently attempts have been made by Clark 

 et al^^ to trace further the course of the stimulus as it 

 passes from the eye to the anterior pituitary gland. In 

 the first place they confirmed that in the ferret the first 

 part of the path lies in the optic nerve. Then they 

 proceeded to make experimental lesions in the brains of 

 female ferrets, and they found that not even the removal 

 of the complete visual cortex could prevent the develop- 

 ment of oestrus at the usual time. The conclusion drawn 

 was that the visual stimulus depends on impulses passing 

 either to the ventral nucleus of the lateral geniculate 

 body, or to the subthalamus via the accessory optic 

 tracts. 



The problem has recently been tackled again by 

 Thomson and Zuckerman^^^ who have once more con- 

 firmed that the stimulus is a nervous one which enters 

 the brain via the optic nerves. 



The important conclusion arising from this work is 

 that the secretory activity of the anterior pituitary is 

 modified by nervous stimulation, and this is confirmed 

 by a study of the psychological factors afltecting repro- 

 duction. In the birds, which have been most extensively 

 studied in this connexion, the eyes again appear to 

 play a dominant role. The stimulus to breeding which is 

 aflforded by the sight of the nesting place, the mate, and 

 sometimes the social group has already been described 

 (p. 41), and this aspect of the problem is well summarised 

 by MarshalU^o in the following words: Tt has been 

 shown that the gonad-stimulating hormone of the 



