514 Cc. A. STEWART 
The brain was next placed upon a moist plate of glass and its 
various parts dissected as follows. The olfactory bulbs were 
removed by a vertical incision at the point where they pass 
beneath the frontal lobes of the cerebrum. The cerebellum was 
then removed by severing its various crura or peduncles. Finally 
the cerebrum was separated from the brain stem by an incision 
immediately anterior to the superior colliculi passing through 
the anterior part of the crura cerebri. The cerebrum thus 
included the telencephalon (except olfactory lobes) and dien- 
cephalon, and the brain stem included the midbrain, pons, and 
medulla oblongata. The four brain subdivisions thus obtained 
were placed in a moist chamber and later carefully weighed in a 
closed container. 
In all cases there is some difference between the total weight 
of the separate parts and the initial brain weight, due no doubt, 
either to evaporation from, or to fluid adhering to, the subdivi- 
sions weighed. Fortunately, however, the error is generally 
small and insignificant, especially as compared with the changes 
in weight that have occurred in the test rats. Based on the 
assumption that the error is probably distributed more or less 
proportionately among the various portions of the brain weighed, 
an attempt was made to correct the existing error n computing 
the percentage for each subdivision, by using the total weight of 
the separate parts rather than the original brain weight. 
The observations in table 1 upon normal rats less than three 
weeks of age are grouped only in instances where there were two 
or more individuals of the same age. Later, when the brain 
growth is less rapid, the data have been averaged, grouping indi- 
viduals differing a few (usually three or four) days in age. In 
the case of the eight adult rats only is there any considerable 
range in ages. Since the sexual difference in brain weight in 
animals of corresponding weight is comparatively small (Don- 
aldson, ’08, ’09), the observations for males and females of each 
group have been combined. This is also justified by the rela- 
tively small number of observations, since the individual varia- 
tions would obscure any existing difference according to sex. 
