138: ELIZABETH HOPKINS DUNN 
fibers. The records for the individual rats are to be found in 
table 1. For the group averages table 2 must be consulted. Of 
the figures fig. 1 gives in Ca curve based upon the averages of the 
groups for the body weight-age comparison, and in curves B and 
A the comparison of age with the size of the largest fibers and of 
their axis cylinders. Fig. 2 gives two curves related directly to 
the number of medullated nerve fibers. Curve D has been plotted 
for the fiber-age complex, and Curve H for group-weight averages 
and number of fibers. Both curves of fig. 2 show considerable 
irregularity in the later stages of growth. There is shown, how- 
ever, a tendency for the number of medullated nerve fibers to in- 
crease in relation to both age and weight. The curves for the 
two are quite different, since the increments of age are daily 
periods, while the body weight is laid on chiefly during early life, 
as is shown by Curve C, fig. 1, by which two-thirds of the maxi- 
mum average body weight is found to be present at one hundred 
and eighty days of life. 
THE NUMBER OF MEDULLATED NERVE FIBERS 
It is a difficult matter to estimate the direct relation of the num- 
ber of medullated nerve fibers in the ventral root of the second 
spinal nerve in the albino rat to the body weight if the attempt to 
do so is made to the exclusion of the age factor. Especially is 
this true in the present series in which the extremes of weight at 
definite ages have been selected. So the attention has been direc- 
ted to the variations to be observed within the group of one age. 
This number-weight relation must be considered for the sexes 
separately. If this is done the statement may be made that of 
two female rats of the same age or two male rats of the same age 
the heavier rat tends to have the greater number of medullated 
nerve fibers. This is especially noticeable when the compared 
rats are of the same litter. Unrelated rats are more likely to vary 
from this rule. This finding in regard to weight corroborates that 
of Mrs. M. H. 8. Hayes in her unfinished work quoted by Hatai 
(08, p. 154) with the added statement that this rule applies to 
the sexes separately. 
