98 Journal of Comparative Neurology. 



2. The Variations in the Number of Fibers found at 



DIFFERENT LeVELS IN THE RoOTS AND TrUNK. 



As before mentioned, these differences are explained on 

 the supposition that fibers growing into the roots and into the 

 trunk have proceeded unequal distances from their cells of 

 origin. 



Ever since the investigations of His ('86 and '89) it has 

 been admitted that the medullated fibers forming the spinal 

 nerves are the outgrowths, on the one hand, of the cells of the 

 dorsal root ganglia, and on the other, of those situated in the 

 gray matter of the spinal cord. 



Kaiser ('91) has shown that the number of mature nerve 

 cells in the cord increases with the growth of the animal, and 

 the work of Birge ('82) is sufficient to show that the fibers of a 

 nerve increase in number during the growth of the frog. Birge 

 found for the frog that the increase in the number of fibers in 

 the spinal nerves was quite proportional to the increase in the 

 weight of the animal. 



Donaldson ('97) determines the fact that the entire neurone, 

 the nerve fiber as well as the cell body, increases in size during 

 the growth of the animal. 



Korybutt-Daskiewicz ('78) observed growing fibers along 

 the sciatic nerve of the frog. This investigator also described 

 the myelin sheath in its formation about the growing axis cylin- 

 der as tapering bluntly and ceasing a short distance behind the 

 growing tip. 



Kolster ('93) in a study of the regeneration of fibers follow- 

 ing the section of the nerve, pictures the sheath as being ac- 

 quired a node at a time and consequently as ending bluntly 

 — the axis cylinder preceding the formation of the sheath. 

 Both observers describe the myelin as first appearing either as a 

 thin layer blackening with osmic acid or as fine irregularly ac- 

 cumulated droplets. Stroebe ('93) observed the sheath as mak- 

 ing its appearance in much the same way. 



