120 S. R. DETWILER 



into the definitive limb bud which centers ventral to the fourth 

 myotome. From this it would appear that the number of seg- 

 mental nerves entering the plexus would be determined by the 

 extent of the limb rudiment at the time when initial connections 

 are made. 



It was Flirbringer (79) especially that called attention to the 

 fact that the nerve plexus from which a limb is supplied might in 

 two cases have a different segmental origin, yet the distribution 

 of the limb nerves arising from the plexus might be exactly the 

 same in each. Both Flirbringer (op. cit.) and Gegenbaur ('98), 

 who closely held to the idea that muscle and nerve form an in- 

 separable unit, admitted the difficulty of satisfactorily explaining 

 such segmental variations. From the results of his own experi- 

 ments, Harrison ('07) interpreted segmental differences as due 

 to the position and extent of the limb rudiment at the time when 

 initial connections of the nerves were made, this serving as an 

 index of the number of nerves contributing to the plexus. From 

 the identity of intrinsic distribution, regardless of metameric 

 origin, Harrison concluded that the mode of segregation and the 

 growth of the structures within the limb determine the specific 

 intrinsic nervous pattern. 



From previous observations (Detwiler, '20) and from the results 

 obtained in the present experiments, considerable evidence has 

 accumulated to show that the position and extent of limb rudi- 

 ments lying beyond the confines of the normal position are 

 not the only factors in determining the source of origin of the 

 nerves contributing to its plexus. This question will be more 

 fully considered in the discussion. 



4- The first and second spinal nerves 



A consideration of the normal pathway of the ventral rami of 

 the first and second spinal nerves is herewith given, since the 

 hypobranchial region is involved in the present experiments. 



The course and terminal connections of the ventral rami of the 

 first and second spinal nerves in Ambly stoma punctatum are 

 found to be very similar to those described by Norris ('13) for 



