92 HENRY H. DONALDSON 
available for examination. The average variability in the 9 
groups from which the 21 litters are taken was 
o = 0.26 + 0.029 and C = 0.32 = 0.035, ; 
while the average of the variabilities of the 21 litters, within 
these 9 groups, was 
o = 0.14 + 0.033 and C = 0.17 = 0.041. 
Thus the variability of the male litters is only about one-half 
that of the age groups in which they are found. 
Among the 11 female age groups, there were 7 which con- 
tained 11 litters of sufficient size for study. Here we find 
much the same relations as appeared among the males. The 
average variability of the 7 female groups from which the 11 
litters were taken was as follows: 
o = 0.25 +0.032 and C = 0.32 +0.041 
while the average of the variabilities of the 11 female litters 
was: 
o = 0.13 +0.30 and C = 0.17 +0.043 
Again the litter variability is about half that of the groups from 
which the litters were taken. 
It appears from the foregoing that the variability of the per- 
centage of water in brains belonging to the same age group is 
small—and that it is about the same for both sexes—but that 
within a given litter it tends to be much less than in the age 
group formed by a combination of the litters 
THE SPINAL CORD 
Although the number of records for the spinal cord is a trifle 
less than the number for the brain, yet all the spinal cords which 
were used are from rats that also furnished brains for the brain 
series. What has been said already (p. 78) in connection with 
the brain, concerning the material and the general character of 
the data, applies therefore to the spinal cord series also. In 
