Takahashi, Internodes on Nerve Fibers. 195 



are relatively short, since they belong to fibers which have been 

 subjected to the lengthening process for only a short time. The 

 presence of these fibers reduces the average length of the inter- 

 nodes, and hence accounts in part at least, for Boycott's observa- 

 tion that on the average the lengthening of the internodes in the 

 sciatic nerve is sHghtly less than that of the nerve itself. It also 

 accounts, in part at least, for the w^ide range in the length of the 

 internodes found on fibers of the same diameter. 



8. In the leopard frog, Rana pipiens, the length of the inter- 

 nodes at the distal end of the sciatic nerve, is on the average, only 

 about two-thirds that of the corresponding internodes in Rana 

 temporaria (fusca) as measured by Boycott ('04). 



SUMMARY. 



The foregoing conclusions may be made more vivid perhaps if, 

 in the light of our present knowledge, we attempt to picture the 

 growth changes which affect the internodes on the nerve fibers of 

 the leopard frog. From the observations of His ('86) Harrison 

 ('01, 'o4-'o6), Bardeen ('o2-'o3), and others, we know that the 

 axone grows out from the cell body into the peripheral nerve, 

 accompanied by its sheathing cells. There are no observations to 

 show whether before the formation of the myelin the sheathing 

 cells cover approximately the same length of fiber in all fibers, or 

 at all periods of growth, but our observations as they stand, would 

 favor such a view. 



In the leg of the tadpole, the formation of myelin occurs first in 

 the fibers which run the shorter course, and interpreting the find- 

 ings of Hardesty ('99) and Hatai ('03) showing a diminishing 

 number of meduUated fibers in the spinal roots as we pass away 

 from the cells of origin, it appears that the development of the 

 myelin progresses from the cell of origin toward the end of the 

 axone. 



When the axone has made its distal connection, and the myelin 

 is formed, then the lengthening begins, and continues so long as 

 the nerve to which the fiber belongs, continues to grow. In the 

 nerves to the leg however this process is modified by the fact that 

 the internodes have a tendency to lengthen at the same rate as the 

 segment of the leg to which they belong; although this process is 

 more marked in the younger than in the older frogs. Despite this 



