82 HENRY H. DONALDSON 
TABLE 1—Concluded 
MALES FEMALES 
Percentage of water Percentage of water 
a Cc ———. —]— KK  — 
Observed | Corrected | In table Observed | Corrected | In table 
310-315 2 78.04 78 .06 Chee) 
315-320 4 77.59 77.53 77.84 
320-325 4 78 .24 77.99 77.74 5 77.92 77.81 17.82 
325-330 
330-335 2 77.95 77.90 CRE (H 
330-340 of 78.09 77.89 77.74 
340-345 3 reel 77.62 rian 
345-350 
300-355 
300-360 5 77.83 77.62 77.62 
360-365 6 77.64 77.49 77.59 
may be considered as in agreement. Where one is making com- 
parisons within a litter or within a homogeneous series, less 
deviation is to be expected and agreement may be limited to 
values that fall within +0.1 per cent of the standard which is 
used. Where data from test animals are contrasted with those 
from controls of the same litter deviations of 0.05 per cent, if 
constant or nearly so, may be regarded as significant. 
Thus far nothing has been said of the way in which the factors 
for correction were obtained or how they have been applied. 
These questions will now be considered. 
Sources of variations in the percentage of water 
If identical in other respects, two brains of the same age should 
have the same water content. Two brains are, however, never 
found to be exactly alike even in the terms of our rather crude 
measurements, and the differences, which we can at present 
appreciate, fall into two classes, those which are gross, and those 
which depend on histological structures. 
1. Variations due to gross differences. ‘There are at least two 
possible causes of variation in the water content dependent on 
gross characters. 
