184 'Journal of Comparative Neurology and Psychology. 



instance, Group (B) where Frog 5 is taken as the standard, the 

 leg in Frog 8 is 15.6 per cent greater. 



Our next step is to make an approximate determination of the 

 increase in the diameter of the growing fibers in the frogs in which 

 the nerves to the leg increase 15.6 per cent over the standard. 



To determine the increase in diameter which probably occurs 

 when the nerve increases 15.6 per cent in length, we proceeded 

 as follows: 



By comparing the sum of the internodal lengths of the 4/«, 5^« 

 and 6/1 fibers in Group I of Boycott's table (reprinted as Table i 

 on p. 168) with the corresponding sum of the 5/1, 6/« and 7// fibers 

 in Group II and these in turn with the sum of the 6/<, 7/i and 8// 

 fibers in Group III, it was found that for an increase of ipt in diame- 

 ter, there was an average increase in internodal length of 25.9 per 

 cent. Since we assume in the case of our own frogs that the inter- 

 nodal length will increase in proportion to the increase in the 

 length of the nerve, and since the latter amounts to 15.6 per cent, 

 it follows that if an increase of 25.9 per cent in internodal length, 

 calls for an increase of ifx in the diameter of the fiber, then 15.6 

 per cent increase in internodal length, will call for approximately 

 0.6/z increase in the diameter of the fiber. 



This result is based of course on Boycott's measurements made 

 on R. temporaria. It seems justifiable to apply it to R. pipiens 

 however because, although Donaldson '08 has shown that the 

 internodes in R. pipiens are shorter than in R. temporaria, he has 

 also shown that the proportional differences in length are nearly 

 the same for the several diameter classes, and hence any given 

 change in the diameter, is associated with the same relative change 

 in length of internode in both species. 



Accepting therefore this determination of the diameter increase, 

 the next step is to compare the internodes on the fibers of a given 

 diameter of one specimen of R. pipiens, with the internodes in 

 another specimen, on fibers which are 0.6/i greater in diame- 

 ter. To do this, we select from the foregoing Tables 5, 6 and 7, 

 the internodal lengths on fibers for the diameter classes 5.3/^, 6.3^ 

 and 7.3,« from all three levels. This permits us to make nine 

 comparisons. 



Thus in each of these comparisons, as for instance in the first 

 one, in Table 10 the average internodal length in the diameter 

 class 5.3/i at S^ in Frog 3, is compared with the average internodal 



