170 G. E. COGHILL 



however, at this level in the coiled-reaction stage (fig. 30). It 

 is made up of a few fibers that are scattered along for some dis- 

 tance ventrad from the cells and immediately against the external 

 limiting membrane of the cord. These fibers are barely percep- 

 tible in cross-section. The more dorsally situated have a per- 

 ceptible inclination ventrad when viewed in the cross section 

 of the cord and the ventrally projecting processes of the giant 

 ganglion cells are in close relation with them. The more ven- 

 trally situated fibers, however, are cut in distinct cross section 

 and form a differentiated longitudinal fiber tract. 



At the level of the third myotome in the early-swimming 

 embryo the sensory tract has extended farther ventrad. In fact 

 it reaches almost or quite to the motor tract {VT) which occupies 

 the most latero-ventral portion of the cord. In its dorso-ventral 

 extension, however, the sensory tract is interrupted here and 

 there by the neuroblasts of other groups. The giant ganglion 

 cells in this case also lie dorsally of the tract and send their 

 processes into its dorsal portion. 



(2) At the level of the eighth myotome. In the non-motile em- 

 bryo, at the level of the eighth myotome, the giant ganglion cells 

 are situated in the dorso-lateral portion of the cord (fig. 32). 

 In the cross-sections there are still no perceptible fibers associated 

 with them. In suitable plane of section, however, a few fibers 

 are demonstrable in a position just ventral of the cells. The 

 direction of these fibers is rostrad and ventrad from the cells and 

 they are obviously attenuated processes from these cells. Al- 

 though not perceptible in transverse section their position has 

 been sketched in on figure 32. 



As compared with the non-motile embryo, that of the early- 

 flexure stage shows a marked development of the sensory tract 

 at this level (fig. 33). The characteristic process of the giant 

 ganglion cell is here seen projecting ventrad towards the tract, 

 the fibers of which are scattered along against the limiting mem- 

 brane to near the motor tract. Study of sections adjacent to 

 the one figured shows that the fibers are distributed sparsely 

 through the region and that cells push in between them and 



