116 S. R. DETWILER 



nerves were derived from the original limb level of the cord had 

 its explanation in the fact that the appendage in such cases 

 was connected with a central nervous mechanism adequate for 

 normal motility. 



The remarkable increased growth of the limb nerves and the 

 entirely new pathways which they followed to reach their normal 

 terminal end organ (limb) when the latter was transplanted 

 considerable distances caudal to its normal position strongly 

 suggested the possibility that the limb might exert some directive 

 influence upon the segmental nerves contributing to its plexus. 



Of considerable significance were the facts that the nerves of 

 the limb level, especially the fourth and fifth, grew greater dis- 

 tances to meet the transplanted limb than did those coming from 

 the more caudal segments, and that the farther away from the 

 normal level the limb was transplanted, the less was its liability 

 to receive nerves from segments situated anterior to the position 

 of the limb, and the greater its tendency to receive nerves from 

 segments corresponding to the position occupied by the limb. 



Experiments on various anuran forms (Braus, '05, and 

 Harrison, '07) have shown, in general, that when a limb is trans- 

 planted to an abnormal (heterotopic) position it becomes in- 

 nervated from that part of the central nervous system of the host 

 corresponding to the position occupied by the implanted limb 

 rudiment. In the majority of these experiments the rudiment 

 was transplanted at a stage when the principal nerve paths were 

 already laid down. Accordingly, when the wound in the host 

 was made for the reception of the transplant, the terminal 

 branches of the nerves in that region were severed and the im- 

 planted rudiment was placed in close apposition to the cut ends. 

 The possibility of the limb's exerting any directive influence on 

 the nerves could hardly be tested in such cases, since, as has 

 already been pointed out, the limb buds were placed in the direct 

 pathway of already formed spinal nerves whose ends were severed 

 in preparing the wound, and it is to be expected that these nerves 

 would continue their growth into the rudiment so placed. 



In the urodele, Amblystoma punctatum, the forelimb rudiment 

 becomes localized quite early, even before the closure of the 



