122 S. R. DETWILER 



nerves. In Amblystoma pimctatum there is normally no com- 

 municating branch to the brachial plexus which is formed from 

 the ventral divisions of the third, fourth, and fifth spinal 

 nerves (fig. 4). 



The above observations concerning the pathways and connec- 

 tions of the first and second spinal nerves are based on a study 

 of serial transverse sections of larvae ranging in age from fifty to 

 seventy days after the closure of the medullary folds. The 

 arrangement of these nerves is not exactly the same in all cases 

 studied, but the variations are of only minor significance. 



EXPERIMENTAL 



The experiments here carried out consisted in transplanting 

 the fore-limb rudiment varying distances anterior to its normal 

 position, with the idea of testing whether the Umb nerves could 

 be induced to change their direction of growth and effect connec- 

 tions with the displaced rudiment. Any effort on the part of 

 the nerves to make connections with the limb so placed would 

 necessitate a reversal of their course as compared with that taken 

 by them when innervating limbs placed caudal to the normal 

 site (fig. 5). Under such conditions the nerves, in order to reach 

 their displaced end organ, would meet with considerable opposi- 

 tion, since the differentiating myotomes tend to direct the seg- 

 mental nerves in a caudoventral pathway. Normal connections 

 made under these conditions would strongly support the idea 

 that there exists a greater attractive influence for the nerves in 

 the developing end organ than in extrinsic structures. 



The experiments were carried out upon embryos in the tail- 

 bud state (fig. 1). The circle ventral to the pronephros (pn) 

 indicates the position of the limb rudiment. The slightly raised 

 eminence just anterior to the limb rudiment constitutes the gill 

 swellings from which develop the three external gills (cf. figs. 

 1 and 9). The position of the first and second myotomes with 

 respect to the gill swellings is also seen. 



