GROWTH OF THE CEREBRAL CORTEX 79 
which is comparable with table 4, the relative value C/D or 
Cortical area 
Cortical thickness 
calculated. The ratio, given in column G, table 5, falls between 
0.85 and 0.99 (average 0.91)? in brains weighing more than 0.5 
gram, but shows a tendency to gradually increase as the brain 
weight increases. In the newborn or before birth (Group I) 
it is somewhat higher. If the average ratio be taken as usable 
for all groups, as in the case of the sagittal section, the cortical 
area in the frontal section may be approximately obtained by 
the following formula: 
W.D xT, X 0.91 (W.D and T., in millimeters) 
As, in brains weighing more than 0.95 gram, the cortical thick- 
ness in the frontal section (7’,) varies only slightly, the cortical 
area in the frontal section in these brains will be practically 
proportional to the frontal diameter of the cerebrum (W. D). 
The agreement of the calculated with the observed values is 
however not so good as in the ease of the sagittal section. 
and the ratio of this value to W. D were 
F. The area of the entire frontal section 
In chart 1, the graph F, representing the total area of the 
frontal section, is accompanied by a dotted line 7’, which repre- 
sents the value of the square of the cube root of the brain weight 
(in grams). Theoretically, under the assumption that the spe- 
cific gravity of the brain remains the same throughout the life, 
the latter should run a similar course to the former, if the brain 
enlarges proportionally in all dimensions as it grows. Between 
Groups II to XIV, both curves take nearly the same course, if 
some slight discrepancies in the observed values are neglected. 
But in brains weighing more than 1.4 grams, the differences 
become so distinct, that they can no longer be regarded as due 
to errors in measurement. This is probably due to the fact 
that the brain is not enlarging proportionally in all diameters, 
? In making comparisons with the Norway rat in part II of this paper, the 
average ratio given by Groups XIII to XX will be that used. This average 
is 0.93. 
