THE NERVOUS SYSTEM OF AMPHIBIA 175 



part of the spinal cord during the period under investigation. 

 These generaHzations may be stated as follows: 



1. The giant ganglion cells and their processes constitute 

 the sensory column of the cord and are the source of all the 

 fibers of the sensory tract. (In the rostral region of the tract of 

 the older stages there may be axones of the descending trigeminal 

 tract, but this question will be critically considered in another 

 paper). 



2. In the more caudal portion of the several ages the column 

 consists of bipolar cells oriented longitudinally in the cord. 



3. In the youngest embryo the same condition prevails in 

 the most rostral portion of the column, but in the older stages 

 the fibers seem to incline more ventrad. 



4. The axones of the cells are directed cephalad into the 

 tract and those arising from more caudally situated neurones 

 assume a ventral position with reference to the axones of cells 

 situated farther cephalad, the tract during this time being a 

 thin sheet of fibers immediately against the external limiting 

 membrane of the cord, intercepted here and there in its dorso- 

 ventral extent by neuroblasts of other groups. 



5. The cells of the column have dendritic processes directed 

 caudad, so that the tract is adapted to conduction rostrad. 



6. There is progressive differentiation in the column, con- 

 sisting chiefly in the growth of axones cephalad. 



7. At the beginning of the period the cells of the column are 

 already differentiated into neuroblasts or functional neurones, 

 that is to say, proliferation of cells probably does not occur in 

 this group during the period under consideration. 



8. The growing, terminal portion of the cord throughout the 

 extent of the tinsegmented mesoderm at least, in so far as it is 

 differentiated from an epithelial condition, is purely sensory, 

 and the sensory column of the region occupies the whole lateral 

 portion of the cord. 



9. Throughout the extent of the trunk the neurones have 

 acquired their typical position and orientation in the cord and, 

 at least in the medial region, have established a sensory tract of 



