146 'Journal of Comparative Neurology and Psychology. 



which is taken as the standard. To do this, we divide the observed 

 values for the R. pipiens series by 1.188, since 57.3 mm., the 

 average length of the sciatic in the series of R. pipiens is 118.8 

 per cent of 48.2 mm., the average length of the sciatic in the series 

 of R. temporaria. 



The observations thus reduced to the same standard are given 

 in the following table. 



TABLE 19. 



Giving the lengths of the internodal segments in ii on the medullated fibers of the 

 sciatic nerve, for frogs with a sciatic length of 48.2 mm., arranged according 

 to the diameter of the fibers. 



* As will be seen, Boycott's value for the length of the internodes in fibers 6-6. 9/i in diameter, is 

 plainly aberrant, and therefore the percentage value for the internodes of fibers having this diameter 

 in R. pipiens, is excluded from the general average. 



The foregoing table shows that when grouped according to 

 diameters, the internodal lengths in R. pipiens range between 64 

 per cent and 71 per cent of that in R. temporaria, the average 

 being 67 per cent. 



It follows from this that R. pipiens has three sheathing cells 

 on a fiber, where R. temporaria has only two, and therefore more 

 cells in the length of the sciatic. 



Consequently R. pipiens has the finer and more complete con- 

 struction, although it is not possible to say what physiological 

 advantage goes with this difference in structure. There are no 

 observations on R. esculenta to compare with those just given. 



Conclusions. From the observations presented, we conclude 

 that the three species studied are similar in general form and 

 proportions, but that R. pipiens has: 



I. A heavier central nervous system. 



