196 Journal of Comparative Neurology and Psychology. 



however the average length of the internodes on fibers of a given 

 diameter increases as does the nerve in which they occur. 



The interpretation of the internodes, as we find them in a sam- 

 ple taken from any nerve, is complicated by the fact that for a 

 long time during growth, new meduUated fibers are appearing. 

 As these new fibers start with very short internodes, and are late 

 in appearing, they have been affected by the lengthening process 

 for a shorter time than those fibers which were completely medul- 

 lated at an earlier date. They must consequently exhibit inter- 

 nodal lengths shorter than would be expected, and since their 

 absolute number increases as the frog becomes larger, and their 

 presence lowers the average length of the internodes at any level, 

 it will necessarily follow, as shown by Boycott ('04), that the aver- 

 age length of the internodes increases a little less rapidly than that 

 of the nerve to which they belong. This is our explanation of Boy- 

 cott's result. 



While this change in the length of the internodes is taking place, 

 there is also a change in the diameter of the fibers. In general, 

 the increase in diameter is in advance of the increase in inter- 

 nodal length, so that, as Boycott has shown, fibers of a given 

 diameter have longer internodes in larger frogs. 



The exact relation of these two processes has still to be worked 

 out, but this relation, depending as it does on the medullation of 

 the fibers at different dates, and on the fact that all fibers of small 

 diameter are not destined to become fibers of large diameter 

 (Boughton '06), but may remain permanently small, seems to 

 account for the great variation in the length of the internodes on 

 fibers of the same diameter, quite aside from the fact that consecu- 

 tive internodes on the same fiber may have very different lengths. 



While the foregoing description is based on the study of the 

 nerve to the frog's leg, we find that it applies also to the growth 

 changes in the roots of the IX spinal nerve, when we take as the 

 Hmits of the dorsal root, the spinal ganglion on one side, and the 

 spinal cord on the other, and in the case of the ventral root, the 

 spinal cord on one side, and the junction point of the ventral and 

 dorsal roots on the other. 



When however we compare the internodal lengths in the IX 

 ventral root with those in the III ventral root of the same frog, 

 taking the same limits, we get the surprising result that the inter- 

 nodal lengths are similar, although the lengthening of the IX nerve 



