l6o 'Journal of Comparative Neurology and Psychology. 



so great a size as the largest of the "large" fibers does during the 

 first fifteen days, or, indeed, so great as most of the "large" fibers 

 do during the first twenty-five days. Between the limits here 

 given, the largest of the "large" fibers increase 504 per cent, in 

 diameter, but between fifteen days and two years it increases only 

 211 per cent., while the largest of the "small" fibers, during the 

 same period (fifteen days to two years) increases 207 per cent. 

 This indicates very clearly that the reason the "small" fibers 

 never attain the size of the "large" is that they come in after 

 the period of most rapid growth. Furthermore, it will be noticed 

 that, in general, the percentage of increase in diameter over the 

 fibers in the 14-gram rat is nearly the same at any given age for 

 the "large" and for the "small" fibers. Hence the rate of growth 

 of a particular fiber does not depend upon the size which it has 

 attained, but upon the age of the animal. 



4. At any age, the area of the sheath in the fully medullated 

 fiber is nearly equal to the area of the axone. Table III shows 

 this relation. The figures w^ere obtained by measuring, at each 

 age, both the entire fiber and axone of ten well-developed fibers 

 in two diameters, and averaging all the diameters obtained for the 

 entire fibre and for the axone. This result corresponds exactly 

 with the results found by Donaldson and Hoke ('05) in a series 

 of vertebrates comprising twenty-six species. 



CONCLUSIONS. 



1. In white rats having body-weights between 11 grams and 

 414 grams, there is a nearly regular increase in the number 

 of medullated fibers in the oculomotor nerve. This increase 

 amounts to 75 per cent. 



2. This cerebral nerve is more nearly complete in the 1 1 gram 

 rat than are the motor roots of the spinal nerves in the same 

 animal. (For the latter Hatai found an increase of 170 per cent, 

 between body-weights of 10.3 grams and 264.3 grams.) Hence 

 the subsequent rate of development of this cerebral motor nerve 

 is much slower than that of the spinal motor roots. 



3. The number of medullated fibers is more closely correlated 

 with the increase in the body-weight than with the mcrease in the 



4. All the medullated fibers continue to increase in size during 



