102 HENRY H. DONALDSON 
SUMMARY 
Evidence has been adduced for the view that both the rela- 
tive and the absolute weight of the brain and the absolute weight 
of the spinal cord, at a given age, are factors tending to modify 
the percentage of water present, in the sense that the heavier 
brain or cord usually shows the smaller percentage of water. 
A presentation has been made also of the data which were 
used as a basis for the formulas by which the percentages of 
water in table 74—here appended—have been determined, and 
of the manner in which the observed values for the percentages 
of water in the brain and in the spinal cord have been corrected 
for the weights of the respective organs. Factors for correction 
have also been given for reducing the observed values for the 
percentage of water in the brain or cord to a form in which they 
may be fairly compared with one another or with the values in 
table 74, when it is desired to use such a table for reference. 
The factors for correction are given on p. 87 for the brain, and 
in table 3 on p. 98 for the spinal cord. 
It is understood, of course, that when an investigator has 
homogeneous data, a comparison of these data with one an- 
other can be perfectly well made without cross reference to a 
table such as that here given. On the other hand, where the 
series of data are from different researches or from different 
strains of rats they should be referred to such a table before they 
are compared with each other. The measures of variability 
have also been found for the percentage of water both in the 
brain and in the spinal cord, and it has been pointed out that in 
both organs the variability is small, but that the variability for 
the cord is about three times that for the brain. Further, it 
appears that the variability within litters is only about half 
that found in the age groups to which these litters belong, a 
relation similar to that already found for the body weight by 
Jackson (713) and by King (’15). The measures of variability 
are given on pages 91-92 for the brain and on pages 100-101 
for the spinal cord. 
In the appendix are reprinted the formulas for the determina- 
tion of the percentage of water in the brain and in the spinal 
