88 HENRY H. DONALDSON 
differences in the absolute size (weight) of the brain Therefore 
by getting the difference in milligrams between the observed 
brain weight and the brain weight for body weight, as given in 
table 74, and multiplying this by 0.0013, a correction was ob- 
tained which could be applied to the crude values for the per- 
centage of water, which appear in table 1. 
From this table there have been omitted, however, both the 
body weights and the observed brain weights for the several 
age groups, so that the final results there given cannot be con- 
trolled except by reference to the original records which are on 
file at The Wistar Institute. 
Application of the correction factor in the case of new data 
To obtain the corrected value for the percentage of water in the 
brain in the case of a new observation, the necessary data are 
the body length, the body weight, the observed brain weight, the 
percentage of water in the brain, and the age and sex of the rat. 
It is necessary also to have access to reference tables which 
give the body weight normal to the body length, and also the 
brain weight and percentage of water (for that brain weight) 
normal to the age. 
With these data it is possible in a given case first to deter- 
mine what correction should be made in the observed percentage 
of water in order to make that value comparable with the per- 
centage of water to be expected when the brain weight was 
normal to the body length. This may be illustrated by an 
example taken from a recent investigation. The data for the 
rat selected are as follows: 
Body weight, 133.5 grams 
Body length, 179 mm. 
Age, 173 days—female 
Brain weight, 1.581 grams 
Percentage of water in brain, 78.61 per cent 
If we turn to table 68 in ‘The Rat’ (Donaldson, 715), it ap- 
pears that for a female rat 179 mm. long a body weight of 144.4 
grams is to be expected. Therefore this rat is under weight. 
