STIMULUS FOR REGENERATION 239 



with one another, it is possible that the equilibrium may be shifted by chem- 

 ical treatments. Thus the chemical constituents of a muscle cell may possibly 

 be rearranged to produce a bone cell. 



Role of nerves 



Another problem in this field — a rather special problem in the case of 

 the regeneration of the limb — is the relationship of the nervous system to 

 regeneration. In our studies on amphibian development we pointed out 

 earlier that the third, fourth, and fifth spinal nerves supply the forelimb. If 

 these nerves are cut in the shoulder region of a larval salamander and the 

 limb is then amputated, no regeneration will take place (Fig. 161). Under 

 these conditions when the nerve supply is lacking the limb undergoes progres- 

 sive dedifferentiation and is completely resorbed, so that all of it is taken into 

 the body. The limb, in other words, completely degenerates after amputation 

 if the nerves are absent. Now the nerves of the intact limb may be cut and the 

 limb will not degenerate, but if the nerves are cut and the limb is then am- 

 putated, the stump will degenerate. 



The general interpretation is as follows: when a limb is amputated the 

 process of dedifferentiation is set in motion and bone and muscle break down. 

 If the nerves are present, these dedifferentiated muscle and bone cells form 

 a blastema. When the blastema is present, it checks or stops this dedifferentia- 

 tion. When the nerves are cut, however, no blastema forms; only dedifferen- 

 tiation occurs, and it continues unchecked. If a blastema is allowed to form, 

 the dedifferentiation of the limb stops. Then the nerves may be cut and 

 regeneration proceeds without nerves. Thus the blastema apparently exerts 

 a controlling influence on the dedifferentiation of the limb. 



These experiments show that in the young salamander the development 

 of the limb — that is to say, regeneration of the limb — is dependent upon the 

 nervous system. And this was not the case in the embryo. The limb developed 

 perfectly well in the embryo without the nervous system. 



Stimulus for regeneration 



Now, finally, we ask: Why does regeneration occur in some forms and 

 not in others? Why do we find limbs regenerating in salamanders and not 

 in other vertebrates ? There is no theoretical reason for the lack of regenera- 



