THE NERVOUS SYSTEM OP AMPHIBIA 173 



(5) At the level of the twenty-third myotome. In considering 

 figure 44 one should note that, owing to the ventral curvature 

 of the caudal region (fig. 56), this section is somewhat oblique and 

 that the dorso-ventral dimension is therefore magnified somewhat 

 out of proportion to the width. The extreme ventral position 

 of the giant ganglion cells in the caudal region, and their prox- 

 imity to the myotomes as well as to the skin is demonstrated 

 here. Only two such cells appear in this figure but adjacent 

 sections show that their distribution is general from the latero- 

 dorsal angle to the latero-ventral angle throughout the caudal 

 portion of the cord in this age. A comparison of figures 56, 40, 

 44, 19, 20, 21 and 22 will help to establish a clear idea of the 

 exact relations in this part of the embryo. 



In this level of the embryo of the early-reaction stage the 

 ganglion cells are still found in the dorsal haK of the cord with 

 their processes extending ventrad. While no fibers can be seen 

 here in cross section, there is in longitudinal section a perceptible 

 tract that is made up of spindle-shaped processes of the ganglion 

 cells. The sensory column is essentially of this same composition 

 in the coiled-reaction and early-swimming embryos (figs. 46, 47), 

 that is to say, it is made up of spindle-shaped processes of the 

 cells and a few fibers that can be detected only in longitudinal 

 section. 



{6) The sensory column at the level of the twenty-eighth myotome. 

 The non-motile embryo drops out of consideration here since it 

 has only twenty-three well differentiated myotomes. In the 

 early-flexure stage at this level the myotomes have assumed 

 a ventral position relative to the spinal cord and the giant gan- 

 glion cells are accordingly found far ventrad in immediate prox- 

 imity to the myotomes. Other sections show them distributed 

 dorsally from this level to the dorso-lateral angle of the cord, 

 though none appears in that region in the figure (48) . The same 

 features of distribution begin to appear, also, in the embryo of 

 the coiled-reaction stage (fig. 49), but in the early-swimming 

 stage the sensory column appears again in the dorsal hah of the 

 cord (fig. 50) . Longitudinal sections show that in all these cases 



