GROWTH OF CEREBRAL CORTEX 9g 
factor in producing this relation is that the Norway brain is 
younger and less advanced in myelination than the albino brain 
of the same weight. 
In table 4 B the cerebral volume of a Norway rat is compared 
with the cerebral volume of an Albino of presumptively the same 
age. Each Norway brain weight group is paired with an albino 
group which has the average brain weight nearest to the corre- 
sponding albino brain weight of the same age with the Norway 
group. The data were all taken from table 3. Compared in 
this way, the Norway cerebrum has a volume about 21 per cent 
above that of the albino cerebrum of the same age, as shown in 
table 4 B. 
The width-length index of the Norway cerebrum, which is ob- 
WwW. Dex 100 
iB 
Group N XI, and decreases as the brain weight advances, drop- 
ping to 97 or less in the last and oldest groups. Compared with 
the like group of the Albino, the width-length index of the Norway 
cerebrum is on the average always higher by 2 or more points 
than that of the albino cerebrum. So, it may be concluded that 
the Norway cerebrum is becoming somewhat elongated as the 
age advances, but not to so marked a degree as does the albino 
cerebrum and that it is always somewhat more rounded in shape 
as compared with the albino cerebrum of the same weight or 
age. The method of measurement here used reveals only in 
part the degree of difference in the shape of the two brains, for 
direct inspection shows the surface of the Norway cerebrum to 
be distinctly more rounded than that of the Albino, especially at 
the frontal poles. 
tained according to formula , is 104 in the youngest 
LITERATURE CITED 
Downaupson, H. H., anp Harar, 8. 1911 A comparison of the Norway rat with 
the albino rat in respect to body length, brain weight, spinal cord 
weight and the percentage of water in both the brain and the spinal 
cord. Jour. Comp. Neur., vol. 21, pp. 417-458. 
Suaira, Naoxt 1917 Comparative studies on the growth of the cerebral cortex. 
I. On the changes in the size and shape of the cerebrum during the 
growth of the brain. Albinorat. Jour. Comp. Neur., vol. 28, pp. 495- 
510 
