SOMATIC MOTOR COLUMN OF AMBLYSTOMA 127 



3. Embryos of the 'coil-reaction' stage 



The origin of root fibers at various levels in embryos of the 

 'coil-reaction' stage is shown in figures 10 to 15, inclusive. 



The root of figure 10 goes to the seventh myotome. The 

 figure is drawn from an approximately frontal section, tipped 

 slightly dorso-laterad on the side figured. There are several 

 fibers in the root, towards the base of which the fibers of the 

 tract arch outward. From one of these a root fiber (VF) clearly 

 arises as a collateral; in the case of another, not clearly shown in 

 the figure, the same mode of origin is practically certain. The 

 opposite root to the seventh myotome is drawn in figures 12 and 

 13 (VF). In this case the root process of the cell extends directly 

 outward through the external limiting membrane, immediately 

 beneath which ascending and descending processes are given off 

 into the tract. Figure 12 shows the focal plane in which the 

 connection of this neurone with the nerve root is clearest, while 

 figure 13 shows the appearance in the focal plane which demon- 

 strates the relation of the neurone to the ventro-lateral tract. 



The root to the eleventh myotome of the same specimen is 

 shown in, figure 11, where the collateral (VF) arises from the 

 descending process of a neurone of the motor column. While 

 the cell body can not be distinctly made out in this section the 

 general form of the process leaves no doubt as to its nature. 

 Again, in this section, the arching outward of the axone at the 

 origin of the root collateral is conspicuous. 



Far caudad of demonstrable ventral roots in this specimen may 

 be recognized the longitudinally oriented neurones of the motor 

 column. One of these neurones, located at the level of the thirty- 

 third myotome, is drawn in figure 14 (VC). This neurone lies in 

 the extreme ventro-lateral region of the spinal cord. Its descend- 

 ing process {DP) is more clearly seen in the section than is its 

 ascending process, though this inequality may be due to the plane 

 of section. The yolk spherules in the basal portion of the de- 

 scending process conform to the polarization of the cell. At this 

 level and in this plane of section the medullary tube is consti- 

 tuted of a single layer of short epithelial cells, excepting as the 



