526 Cc. A. STEWART 
SUMMARY 
1. The weights of various parts of the brain were studied in 64 
normal rats at various ages, and also in 29 test animals, of which 
15 individuals were held at birth weight by underfeeding for 
periods ranging from 5 to 18 days of age, 13 rats were permitted 
to increase slightly in weight reaching approximately 10 grams at 
3 weeks, and one male weighed 12 grams at 56 days of age. 
2. According to the data available, the cerebrum (excluding 
olfactory bulbs) increases from a normal average of slightly 
more than 64 per cent of the entire brain at birth, to a maximum 
of about 71 per cent during the early part of the second week, 
but subsequently forms a progressively smaller proportion of the 
brain, decreasing to an average of about 61 per cent in the adult. 
The brain stem (including midbrain, pons, and medulla 
oblongata) decreases from a normal average slightly exceeding 
29 per cent of the brain at birth to about 16 per cent at 3 weeks, 
but later increases reaching a relative weight of about 22 per 
cent in adult animals. 
The cerebellum increases rapidly from an apparent average 
of 3.7 per cent of the entire brain at birth to about 14 per cent 
at 7 weeks of age, and thereafter. 
The olfactory bulbs, while variable, in general increase from 
an average of about 2.5 per cent at birth to 3.7 per cent at 3 
weeks, and probably reach a relative maximum of slightly more 
than 4.5 per cent at 6 or 7 weeks of age. Subsequently there is 
a gradual decrease in relative weight to about 2.9 per cent in the 
adult rat. The data indicate not only a relative decrease, but 
even an absolute loss in the weight of the olfactory bulbs in the 
older rats. 
3. For the stunted rats the weight of the brain as a whole in 
the individuals held at birth weight for various periods averaged 
114 per cent higher than that in the controls, whereas the excess 
in test rats weighing 10 grams at 3 weeks and 12 grams at 8 weeks 
of age was 33 and 30 per cent, respectively. 
As shown in figure 1, the increase is shared not only by the 
various brain parts dissected and weighed, but also by the ol- 
factory tracts, tuber cinereum, colliculi, and the paraflocculi. 
