SOMATIC MOTOR COLUMN OF AMBLYSTOMA 131 



Figure 27 is taken from the level of the tenth myotome of 

 the same specimen as that of figure 24. Here, again, the loop 

 of the tract fibers is perceptible, particularly in the case of the 

 fiber 'from which the collateral (VF) arises. This root fiber 

 arises from the rostral arm of the loop, while there can be no 

 doubt that the caudal arm of the loop extends on caudad into 

 the tract. The appearance of the fiber at the branching would 

 indicate that the fiber from which this root collateral arises is 

 a descending process of a tract neurone. This is the case also 

 in figure 22, which is drawn from the level of the ninth myotome 

 of an embryo of the same stage of development. The arching 

 outward of the tract fibers at the origin of the root is apparent 

 here also, and particularly is this true in figure 23, which is 

 drawn from a different focal plane of the same field. The dis- 

 appearance of the more mesial fibers of the tract in the middle of 

 the loop as shown in figure 23 is caused by their passing out of 

 the plane of section as they deflect from their regular longitu- 

 dinal course in the tract. This deflection in the fibers of figure 

 23 is directly towards the origin of the root in figure 22. In 

 the case of figure 21, which is drawn from the root of the eighth 

 myotome of the same embryo as the last, the root collateral has 

 the appearance of arising from an ascending process. The plane 

 of section, however, is here very oblique and the exact relations 

 difficult of interpretation. The root fiber arises from a tract 

 fiber which appears to be the axone of the cell marked VC, and 

 the direction of this main process is almost if not quite laterad. 

 The relations here are probably the same as those demonstrated 

 by a different method in figures 12 and 13. 



The most advanced stage of development of the ventral root 

 system under consideration is represented in figure 28, which is 

 drawn from the first ventral root of an embryo of the early swim- 

 ming stage. In this case a densely impregnated fiber which lies 

 deeply embedded in the ventro-lateral tract arches outward 

 slightly towards the root of the first spinal nerve and gives off 

 a collateral (VF) which passes directly laterad among the fibers 

 of the tract and enters the nerve root. 



