THE NERVOUS SYSTEM OF AMPHIBIA 167 



Another noteworthy feature in the distribution of the giant 

 ganghon cells is conspicuous in these projections, namely, that 

 the cells diminish in numbers rapidly from the level of about the 

 fourth post-auditory myotome rostrad and assume a more ven- 

 tral position in the spinal cord, while in the more caudal regions, 

 also, there is a marked shifting of the cells ventrad. This dorso- 

 ventral position of the cells in the rostral region might be ex- 

 plained as due to the proliferation of other cells dorsally, in 

 relation to the development of the nuclei of the dorsal funiculi 

 and allied structures; but no such explanation can apply to 

 the ventral position of the cells in the caudal parts of the 

 cord. In the latter region the ventral position of the cells is 

 distinctly allied with the position of mesoderm, as the following 

 description of cross sections will show. In fact, when the entire 

 spinal cord is taken into account, the dorso-ventral position 

 of the giant ganglion cells shows a more distinct correlation with 

 the position of the mesoderm than it does with any structures in 

 the cord. In following the further descriptions, one should keep 

 this point in mind. 



6. The giant ganglion cells as a sensory column 



For the purpose of illustrating the relations of the giant gan- 

 glion cells within the spinal cord as a sensory column and the 

 processes of differentiation that can be recognized in the system 

 during this period of development figures 28 to 54 have been made 

 with the aid of a drawing apparatus at a magnification of four 

 hundred diameters (reduced to two hundred in the figures). 

 This series of drawings is intended for use in future descrip- 

 tions of the growth of the spinal cord and with this in view the 

 other structures in the sections have been drawn with as much 

 care as has been given to the giant ganglion cells. No detailed de- 

 scription of the other features of the cord will be given here, 

 but a cursory examination of the figures will enable one to appre- 

 ciate somewhat the setting of the functional sensory column in 

 the midst of a mass of rapidly proliferating and differentiating 

 cells which belong to other groups. 



