SOMATIC MOTOR COLUMN OF AMBLYSTOMA 125 



division, apparently the larger in this case, passes out of the cord 

 as a root fiber (VF) and reaches almost to the anlage of the 

 spinal ganghon {SG). The other division continues caudad in 

 the latero-ventral tract. Descending processes of other cells enter 

 this root, as shown in the figure, without giving any evidence 

 of bifurcation beneath the limiting membrane, but there is noth- 

 ing in the preparation that would render such a branching improb- 

 able. In fact, slightly caudad of the root are clearly differentiated 

 fibers of the tract which may well be the descending processes 

 of these same neurones. 



Figure 4 is drawn from the opposite side of the same embryo 

 and the section is therefore tipped slightly latero-ventrad. It 

 shows a root fiber (VF) arising clearly as a collateral of a tract 

 fiber and passing through the anlage of the spinal ganglion to 

 the muscle cell {MC). Its ending upon this muscle cell can not 

 be regarded as clearly demonstrable in this section but the fiber 

 expands slightly upon the surface of the cell and apparently ends 

 there. Just caudad of the region of this root collateral is the 

 basal portion of another fiber which almost certainly has the same 

 relation to the neurone of the tract. The origin of these root 

 fibers from the tract opposite the middle of the myotome and 

 their application to the muscle cell directly opposite a large 

 nucleus against which impinges a conspicuous mass of pigment 

 (P) is characteristic of these early neuro-muscular relations. The 

 outward thrust of the axone of the tract at the point of origin 

 of the root collateral is also characteristic, as shown in figure 4. 



The form of the neurones of the motor column is illustrated in 

 figure 5 (FC). The neurone of the figure occurs at the level of 

 the thirteenth myotome, some muscle cells of which (MC) are 

 shown in the figure. The position and general relations of this 

 neurone in the cord are shown in figure 6 (VC), from which the 

 plane of section of figure 5 can be readily interpreted. Although 

 the portion of the cell visible in this section is only about half 

 the length of the adjacent myotome, the entire cell probably 

 exceeds the myotome in length. Another neurone of this column, 

 located at the level of the twenty-first and twenty-second myo- 

 tomes (fig. 8, VC,M) is shown in figure 7, and its general rela- 



