240 



EMBRYOLOGY 



Fig. 161. The influence of nerves on limb regeneration. Left: A larval sala- 

 mander with both forelimbs amputated through the radius and ulna. The nerves 

 to the left forelimb stump are intact, whereas those of the right forelimb stump 

 have been cut as indicated. Right: Some weeks later the left forelimb has 

 regenerated, whereas the right forelimb has regressed. The nerves to the right 

 forelimb must be cut repeatedly or they themselves will regenerate and enter the 

 stump. If they do so then the stump regenerates instead of regressing. The precise 

 effect of the nerves in promoting regeneration is still unknown. 



tion in other forms. Moreover, if we apply the principles of early develop- 

 ment, it is possible to stimulate regeneration in forms which never normally 

 regenerate. And this is important, because it gives us a control over regenera- 

 tion. 



In the discussion of early development it was shown that for each step in 

 development a stimulus of some sort is necessary. For the initiation of de- 



