PARTS OF BRAIN IN NORMAL AND UNDERFED RATS 523 
-rats (table 2) is compared with that in normal brains (table 1) 
of the same weight, however, the agreement is surprisingly 
close. It is therefore apparent that during the persistent growth 
of the brain in underfed young rats the cerebellum, like the seg- 
ments previously considered, maintains approximately the same 
relative size as in the normal brain of corresponding weight. 
OLFACTORY BULBS 
Calculations from my data (table 1) indicate that the olfactory 
bulbs, although rather variable, in general increase from an 
approximate average (sexes combined) of 2.5 per cent of the 
total weight of the separate parts of the brain at birth, to about 
3.7 per cent at three weeks, and probably reach a relative max- 
imum of about 4.8 per cent at six or seven weeks of age. Sub- 
sequently their relative weight decreases in the majority of 
cases, reaching about 2.9 per cent in the adult. Attention should 
be called to the fact that the data indicate not only a relative 
decrease, but even an absolute loss in the weight of the olfactory 
bulbs in the older rats. In general my results agree with those 
obtained by Hatai (15), Sugita (17), Holt (17), and Donaldson 
(unpublished data) for normal albino rats, the existing differ- 
ences probably being due partly to normal variability and partly 
to experimental error. 
In the stunted rats the olfactory bulbs greatly exceed those 
in the younger controls of the same weight (table 2). For the 
group held at birth weight there is an increase of nearly 240 per 
cent. For the other test animals at three and eight weeks of 
age the increase is less marked, amounting to 30 and 71 per cent, 
respectively. 
The percentage that the olfactory bulbs form of the entire 
brain weight, especially in the case of the individuals kept at 
birth weight for various periods, averages higher than that for 
the newborn controls. According to my data, accompanying an 
increase in brain weight from approximately 0.2100 gram to 
0.4500 gram there is normally a considerable increase in the 
relative weight of the olfactory bulbs, although apparently not 
so great as in the stunted rats with brains of corresponding weight. 
