THE NERVOUS SYSTEM OF AMPHIBIA 189 



1. THE REFLEX MECHANISM 



In my earlier paper on Diemyctylus ('09) it was shown that 

 trunk movements in response to stimulation upon the caudal por- 

 tion of the trunk as well as to stimulation upon the head are 

 initiated in the rostral portion of the musculature and that they 

 progress from this region caudad as a wave of contraction through 

 the myotomes of one side. While it is difficult or impossible to 

 detect this characteristic of movement in amphibian embryos 

 that develop rapidly and move quickly, the same cephalo-caudal 

 progression of contraction clearly occurs in Amblystoma as has 

 been described for Diemyctylus. 



This feature of behavior is not only explained but made neces- 

 sary by the fact that the motor innervation of the myotomes is 

 by collaterals of neurones that constitute a continuous motor 

 column in the spinal cord of the embryo, as shown in my paper on 

 this subject ('13). My anatomical studies establish such a motor 

 tract and column in the latero-ventral portion of the spinal cord. 

 Numerous of my experiments corroborate these anatomical 

 findings. By piercing the embryo, for instance, or by cutting it 

 through from the ventral side so as to sever the ventral part of 

 the cord while the dorsal portion is left intact, cephalo-caudal 

 conduction, as evidenced by muscular contraction, has been in- 

 tercepted without interfering with conduction caudo-cephalad 

 past the lesion. In like manner, by inflicting a cut into the dorsal 

 part of the cord, conduction caudo-cephalad has been intercepted 

 without interference with the conduction cephalo-caudad past 

 the lesion. In the former case, the muscle wave affects only 

 the part cephalad of the lesion when the stimulus is caudad of 

 the lesion; in the latter case, the entire trunk contracts in response 

 to a stimulus that is applied cephalad of the lesion while no re- 

 sponse at all can be elicited by stimulation caudad of the lesion. 

 These experiments, although they do not establish the exact 

 dorso-ventral extension of the tracts, certainly furnish ample 

 physiological corroboration of my anatomical results, namely, that 

 the most ventral part of the cord is motor and that the dorsal 



