SPINAL NERVES IN VERTEBRATES 183 



ently Polistotrema agrees with Petromyzon in that the size of 

 the motor fibers is in direct proportion to the number of muscle 

 fibers each must supply. Also the following observations point 

 to the fact that some of the large motor fibers are the axones of 

 the giant Miillerian cells of the spinal cord: (1) The series from 

 which reconstruction 5 was made was stained rather deeply 

 with iron haematoxylin to bring out the nerve fibers in the cen- 

 tral nervous system. As a result both the large motor fibers 

 of the spinal nerves and the giant or Miillerian fibers of the 

 central nervous system were stained a much deeper brown than 

 the sensory axones. (2) Upon examining sections of different 

 levels of the spinal cord it was found that the number of giant or 

 Miillerian cells and fibers became greatly reduced in the tail 

 region in direct proportion apparently to the reduction of the 

 body musculature. In a region behind the caudal heart there 

 were so few Miillerian fibers that it was possible to make a series 

 of reconstructional drawings of a given cell and its fiber. The 

 cell selected was situated in the ventral border of the flattened 

 gray matter about equidistant from the central canal and the 

 lateral surface of the spinal cord, in a region which is undoubt- 

 edly somatic motor or effective. This cell had at least six very 

 large paired processes. Two lateral processes, evidently den- 

 drites, were followed for short distances dorsally and ventrally 

 into the gray matter. Two ventral dendrites traversed the 

 ventral white matter to the ventral surface of the cord. Of the 

 two median processes, a dendrite passed through the ventral 

 white matter nearly to the center of the cord, while the remain- 

 ing process or neurite assumed a general caudal course in the 

 white matter, about midway between the central canal and the 

 lateral surface of the cord. Throughout its course through 48 

 sections of 15 microns thickness it became gradually reduced 

 in caliber and assumed a more ventral and lateral position. 

 When last seen it was a little median of some ventral root fibers, 

 which it probably joined. In tracing out this fiber a second 

 giant cell was observed. Its position was more dorsal than the 

 first, being located in the border zone of the dorsal gray and white 

 matter. 



