158 Journal of Comparative Neurology and Psychology. 



representing an age at which increase in the number of fibers 

 practically ceases. 



An inspection of these tables and figures shows the following 

 points: 



I. There is a nearly regular increase in the number of medul- 

 lated fibers in rats while growing from 11 grams to 414 grams in 

 body-weight. This increase is most rapid in young rats, and be- 

 comes slower as they get older; after sixty days ( = 100 grams) it is 

 very slow. The increase between the limits here given (eleven days 

 to two years) amounts to 75 per cent, of the total number of fibers. 

 (After the age of fifteen days the increase in the number of "large" 

 fibers probably is not significant. The relation between the 

 "large" and "small" fibers will be discussed later.) Hence this 



TABLE III. 

 Ratios Between the Areas of Sheath and Axone. 



cerebral motor nerve is more mature in a rat weighing 1 1 grams 

 than are the spinal motor roots, and it develops more slowly 

 during after life. As already stated. Dr. Hatai ('03) found in 

 rats having body-weights between 10.3 grams and 264.3 grams an 

 increase of 170 per cent, in the ventral roots of the spinal nerves. 

 A curve based on his enumerations is plotted for comparison with 

 my curve in Fig. i. 



2. The increase in the number of meduUated fibers is corre- 

 lated with the weight rather than with the age of the animal, for 

 the curve relating the number of fibers to the body-weight (solid 

 line, Fig. i) is smoother, in general, than one based on the age 

 (broken line, Fig. 2), although two rats (both one hundred and 

 thirteen days old) show an unusually large number of fibers. 



