THEORIES OF CONTROL OF NERVE OUTGROWTHS 



149 



TRANSPLANTED NORMAL SITE 



LIMB BUD OF LIMB BUD 



NORMAL LIMB NERVES 



2 



6,7,8 NERVES SUPPLY TRANSPLANTED LIMB 



Fig. 87. The transplanted forelimb fails to direct the third, fourth, and fifth 

 nerves toward itself if it is placed too far away from the origin of these nerves. 

 In the position indicated the limb becomes innervated by the sixth, seventh, and 

 eighth spinal nerves. 



and when they reach it they turn back and enter the limb. Therefore, there 

 seems to be a general stimulus, a general attraction, exercised by growing 

 organs for nerves. It does not seem to make any difference whether this 

 general attraction is exercised by the limb or by the nasal placode. The nerve 

 fibers of the limb do not, however, penetrate a foreign object. Thus there is 

 a specificity which determines the penetration of nerve fibers into a tissue, 

 and this is independent of the attractive forces. 



Theories of control of 

 nerve outgrowths 



What could be the nature of this general attraction exercised by all 

 growing structures? Three theories or three interpretations have been sug- 

 gested. There may be a chemical attraction exercised by the limb over these 

 growing nerves. Another suggestion is that the directive force is an electrical 

 force. Finally, there may be a mechanical direction of the nerve fiber. 



