78 HENRY H. DONALDSON 
be treated separately, and for each there will be given in special 
tables the observed values for the percentage of water, followed 
by the values corrected for weight, and finally by the formula 
(or table) values. Using the uncorrected values, the measures 
of variability in this character will also be given. 
THE BRAIN 
The data and their treatment 
The data used for table 74 (Donaldson 715) have been gradu- 
ally accumulated during the past eight years, and were collected 
by random sampling, mainly from the rat colony at The Wistar 
Institute. The animals used represent, therefore, the general 
rat population of the colony living, at successive periods, under 
somewhat varying dietary conditions. In many cases, the rats 
were small for their age. Although a number of litters are in- 
cluded in the series, the majority of the animals were not closely 
related to each other. 
The records forming this series run from birth to 365 days of 
age, and may be grouped as follows: 
572 male brains—comprised in 61 age groups 
375 female brains—comprised in 61 age groups 
About one-half of the records here used were collected for an 
earlier study (Donaldson 710), and the remainder have been 
gathered since that time. 
The method of removing and preparing the brain and the spinal 
cord has been described earlier (Donaldson 710). Most of the 
removals were made by my colleague, Dr. Hatai, and the pro- 
cedure has been uniform. ‘The drying was done in a large water 
bath at the temperature of 92°-97°C. We have no reason to 
think that the observational errors have been important under 
these conditions. 
The accompanying chart 1 shows, for the males only, the course 
of the percentage of water from birth to 365 days, in both the 
brain and the spinal cord. The continuous graphs are based 
on the formulas (pp. 104-105) and the separate entries give the 
observed values—corrected for weight—for the percentage of 
