172 G. E. COGHILL 



In the early-swimming embryo the sensory tract appears 

 clearly in cross section as shown in figure 39. It extends here 

 from just dorsad of the motor tract to the ventral projections of the 

 giant ganglion cells. 



(4) At the level of the eighteenth myotome. At the level of the 

 eighteenth myotome in the non-motile embryo the spinal cord 

 frays out dorsally into neural crest structures (fig. 40) . Scattered 

 from the dorso-lateral angle of the cord ventrad to the level of 

 the dorsal margin of the myotome are several of the giant gan- 

 . glion cells, which are conspicuously differentiated from the 

 other cells of the vicinity. In longitudinal sections cells in this 

 position are found to have a definite orientation with their long 

 axis longitudinal in the cord. Almost the entire cord, therefore, at 

 this level is sensory. Some of the ganglion cells are in proximity 

 to the skin, while others are quite as intimately related to the 

 myotomes. These features of the sensory column will be brought 

 out in detail in the latter part of the description. 



Comparison of figure 41, which is at the corresponding level 

 of the embryo of the early-flexure stage, with figure 40 shows 

 a great change in the general features of the cord and in the posi- 

 tion of the giant ganglion cells. The sensory column here occupies 

 the dorsal half of the cord. No sensory fibers can be detected 

 in it by the study of cross sections, but in longitudinal sections, 

 a distinct fiber tract appears at the ventral end of the ganglion 

 cells. Its position is sketched in the figure. The fibers of this 

 tract must be short, however, for if they had great length they 

 would be found farther ventrad in accordance with the relations 

 in the more rostral portions of the cord. 



In the corresponding level of the c oiled-reaction stage there 

 is still no perceptible fiber tract, but, again, in suitable plane there 

 appears a sensory tract which is more clearly differentiated than 

 the motor tract at the same level (fig. 42). In the embryo of the 

 swimming stage the sensory column still occupies the dorsal half 

 of the cord, and a giant ganglion cell is seen far ventrad (fig. 43). 

 The sensory tract here is more clearly differentiated than in the 

 younger embryo. 



