GROWTH OF THE CEREBRAL CORTEX 81 
is almost equal throughout all brain weight groups, swinging 
within the narrow limits of 0.26 to 0.30. 
G. Percentage of the area of cortex in the total area of the frontal 
section (one hemicerebrum) 
Figure 2 shows the outline of the frontal section. In the 
section we see as the principal divisions the cortex, the striatum, 
the thalamus, the capsula externa and the lateral ventricle, 
and, among these, the cortex and the striatum stand in marked 
contrast. In the young brains, the lateral ventricle is wide. 
This cavity was not included in the measurement of the area. 
In the wall of it, especially at the dorso-lateral corner, there 
are seen masses of dividing cells and of neuroblasts, which are 
due to migrate into the cortex. But in the older brains weigh- 
ing more than 1.1 grams, the ventricular wall is almost free 
from dividing cells and the cortex is no longer receiving new 
cells. By determining the percentage of the cortical area to 
the total area of the frontal section, we may obtain some clue 
as to mass relation of the cortex to the other structures seen in 
the frontal section. 
As previously given in table 2, the cortical area in the frontal 
section amounts to 34 per cent of the total area at birth. It 
increases from birth up to brains weighing 0.7 to 1.2 grams, 
when the percentage reaches its highest figure, that is, 50 or 
sometimes 51, on the average 48 per cent. After this stage, 
the percentage slowly diminishes as the brain weight increases, 
and, at full maturity, it reaches 44 per cent or less; even 39 per 
cent in an old brain weighing more than 2.0 grams. This means 
clearly that the cortical area increases rapidly by receiving 
new cells from the matrix and at the same time by the enlarge- 
ment and separation of the cell bodies, during the first phase, 
covering the first ten days after birth. 
In this phase, as a matter of fact, the remainder of the sec- 
tion is for the most part composed of the matrix and migrating 
cells, the central nuclei being not yet so largely developed. 
The transitional layers, or the areas previously occupied by the 
