152 S. R. DETWILER 



shoulder musculature and imperfect innervation as is seen in 

 table 3 is due no doubt to the active gill-forming properties of 

 the surrounding ectoderm, which in most cases either inhibited 

 the formation of the limb or exerted disturbing influences upon 

 its differentiation. The latter was manifest by considerable 

 torsion in the developing appendage and a high percentage 

 of abnormalities (table 1). 



Examination of sections showed that even when the limb was 

 structurally complete, the shoulder, in many cases, was quite 

 distorted. The resultant abnormalities were found to be greater 

 in the musculature than in the girdle itself, which was quite 

 extensively developed in some cases. 



Lack of freedom in the motility of the limbs was also due in 

 part to the inhibitory influence exerted by the gill ectoderm which 

 frequently extended out over the proximal portion of the arm 

 (case AA2S25, fig. 14). The arm in such cases was not only 

 fused to the side of the body, but the shoulder-joint was deeply 

 situated beneath the surface, thus mechanically preventing 

 freedom in its movements. 



DISCUSSION 



From the results of the foregoing experiments, in addition to 

 those previously obtained (Detwiler, '20), evidence has accumu- 

 lated which strongly indicates that there exists between the limb 

 and its normal nerves a developmental relationship which is 

 more intimate in character than any developmental association 

 between these same nerves and other structures. This is sug- 

 gested by the apparent tendency on the part of the hmb nerves 

 to effect functional connection with their proper end organ (limb) 

 when the latter is shifted a given number of segments posterior 

 or anterior to the normal site (table 2 and figs. 5, 8, 16, and 18). 



All the previous observational evidence tended to support the 

 idea that the number of segmental nerves contributing to the 

 normal limb plexus is governed by the position and extent of the 

 limb rudiment at the time when it occupied its maximal extent 

 in the embryo, and that plexus formation itself results from 



