SOMATIC MOTOR COLUMN OF AMBLYSTOMA 129 



tal plane. In the ventral part is seen the ventro-lateral tract 

 {VT) into which the broad processes of the neurones of the motor 

 column (VC) project. In the dorsal region appear the perikarya 

 of the giant ganglion cells which form the dorsal sensory column. 

 The nuclei in this figure are all drawn with the aid of the camera 

 lucida. A general idea, therefore, can be formed from this draw- 

 ing of the relative size and similarity in arrangement of these 

 two types of neurones. Here the nuclei of the motor column are 

 distinctly smaller than those of the giant ganglion cells, and my 

 observations lead me to believe that they are generally so. It is 

 noteworthy also that while there is a sharp differentiation of the 

 giant ganglion cells from all others around them, there is more 

 or less of a gradual transition from the typical, large, round 

 nucleus of the motor column and the elongated nuclei of the 

 ependyma. A corresponding gradation in nuclei is also seen in 

 certain staining reactions. Such difference in differentiation is 

 obviously correlated with the difference there is between these 

 motor and sensory cells in the further development of the animal. 

 The giant ganglion cells are transitory and do not become perma- 

 nently worked into the definite organization of the spinal cord, 

 while the motor cells are part of a developing system which 

 elaborates with the development of the animal. The motor sys- 

 tem, therefore, is here a developing system whereas the dorsal 

 ganglion cell system has reached its full development so far as 

 cell differentiation is concerned. 



5. Embryos of the early sivimming stage 



The general relation of the motor and sensory neurones of the 

 cord in an older specimen is shown in figures 18 and 19. Figure 

 19 is drawn through the cord at the level of the ninth myotome, 

 while figure 18 is taken from the level of the eighteenth myotome. 

 The ascending process of the giant ganglion cells (DC) may be 

 seen here projecting slightly ventrad toward the dorso-lateral 

 tract (DT). The neurones of the motor column (VC) lie mesially 

 or slightly dorsally of the ventro-lateral tract (VT). Their 

 peripheral ends project into the tract and bifurcate into ascend- 



