90 HENRY H. DONALDSON 
weights and brain weights of the female are regularly less than 
those for the male. It we determine for a series of cases the 
difference in the percentage of water between the male and 
female brains of like age, we find that a difference of 1 mgm. in 
brain weight corresponds to a water difference of about 0.001 
per cent, thus giving an amount which is a trifle below the cor- 
rection factor for the brain within each sex. It seems prob- 
able that the perikarya of the neurons in the male are relatively 
somewhat larger than in the female, and this would account 
for the slightly lower value found by this method of comparison. 
Measures of variability in the percentage of water 
The material represented by the data in table 1 makes it 
possible for the first time to determine the variability of the 
percentage of water in brains of like age, not only when the 
brains are taken by random sampling, but also when they be- 
long to a single litter. The measures of variability determined 
for each sex were the standard deviation (c) and the coefficient 
of variability (C). 
For the determination of the standard deviation o we used 
the formula 
| MEGS) 
n 
and for the probable error of the standard deviation 
oO 
EL, = + 0.6745 Wa 
n 
For the coefficients of variability, C, the formula 
= 7 x 100 
and for the probable error of the coefficient of variability 
E, = + 0.6745 a 
; V 2n 
(Davenport 04). 
