THE NERVOUS SYSTEM OF AMPHIBIA 171 



against the limiting membrane of the cord. In the section from 

 which figure 33 was taken only the fibers figured nearest the 

 giant ganglion cell could be detected. Those pictured farther 

 ventrad in the figure were sketched in from nearby sections of the 

 same series. 



In the embryo of the coiled-reaction stage (fig. 34) the sensory 

 tract is perceptibly strengthened as compared with that in 

 figure 33. It appears as a continuous sheet of fibers from the 

 motor tract dorsad to the giant ganglion cells, which are in this 

 stage, also, located dorso-laterally. Just dorsad of the motor 

 tract the sensory tract narrows down and disappears. In the 

 early-swimming embryo (fig. 35) the sensory tract is difficult 

 to outline clearly in this particular section on account of the 

 neuroblasts which have invaded its territory, but in the opposite 

 side of the cord in this section and in near-by sections of the series 

 the sensory tract shows the same general relations as in figure 34, 

 except that it is more strongly developed in the older embryo. 

 The process of the giant ganglion cell in the figure reaches to 

 the dorsal border of the sensory tract. 



(S) At the level of the thirteenth myotome. At the level of the 

 thirteenth myotome the giant ganglion cells still occupy a dorso- 

 lateral position in the cord of all four embryos (figs. 36, 37, 38, 

 39), and their processes extend towards or into the tract. In 

 transverse section no distinct fiber tract can be detected at this 

 level in the non-motile embryo, but in favorable planes of sec- 

 tion spindle-shaped and attenuated processes can be traced 

 from the giant ganglion cells into an indefinite tract of short 

 fibers. Its position is indicated in figure 36. In the embryo of 

 the early-flexure stage (fig. 37) a giant ganglion cell appears in 

 the lateral region of the cord, midway from the dorsal to the 

 ventral borders. No sensory tract can be clearly differentiated 

 in transverse section. In embryos of the coiled-reaction stage 

 also no sensory tract can be detected in transverse sections. 

 In favorable planes of section, however, there is unmistakable 

 evidence of a small tract of fibers in the position indicated in 

 figure 38. 



