HORMONES AND REPRODUCTION 75 



showed some response to the whole of the visible spec- 

 trum. ^33 Again the experiments showed that animals 

 differ widely regarding the factors to which they have 

 become attuned. 



The next step in the analysis was to study the pathway 

 along which the action of the light passes into the body. 

 This problem was considerably simplified by the obvious 

 expectation that the only organ capable of appreciating 

 increases in light is the eye, while the one internal organ 

 which must be stimulated is the anterior pituitary gland. 

 The problem was therefore to discover the route between 

 them. 



To prove the importance of the eye itself a number of 

 experiments were performed, the first being those of 

 Benoit.27-29 He worked with the duck and showed that 

 the removal of the eyeballs did not inhibit the growth of 

 the gonads in spring. However, when ducks were kept in 

 darkness the spring gonad development was slower than 

 usual, and if hoods were placed over the head a similar 

 result was obtained. From this last observation he con- 

 cluded that the region round the eyes of a duck is also 

 light sensitive so that if either the eyes or this surround- 

 ing region is left exposed the gonads mature at their 

 normal rate. It now appears that the duck, a bird that 

 is highly bred for domestic purposes, is an unsuitable 

 animal for such work since it apparently possesses a 

 powerful inherent reproductive rhythm by which it is 

 able to achieve breeding condition in the complete 

 absence of external stimulation. A similar criticism 

 applies to the work of Ivanova^i* who used the sparrow, 

 which moreover has only a diffuse breeding season. 



More reliable results have been obtained from the 

 ferret. In this species the internal rhythm is not so 

 strong, and in the absence of light in spring sexual 

 development is either prevented or greatly delayed, ^^a 

 Experimentally it has been shown that the cutting of the 

 optic nerves has the same effect in eliminating or delay- 

 ing the onset of heat. Thus the importance of the eye 



