44 THE RIDDLE OF MIGRATION 



by the administration of testicular extracts. The 

 behaviour ceases if the extracts are discontinued. 

 These experiments, while they demonstrate the 

 dependence of behaviour on the gonad, or part of 

 the gonad, do not necessarily substantiate the claim 

 that the interstitial cells are responsible. The point 

 has, however, been demonstrated in mammals where 

 the sex cells have been destroyed by means of 

 X-rays leaving the interstitial tissue unimpaired. 

 Under such conditions the animal retains all its 

 male characters and behaviour, although it is now 

 sterile and no longer a functional male. 



