GROWTH OF THE CEREBRAL CORTEX Je 
X. SUMMARY 
1. On the sagittal and the frontal sections from 28 Norway 
rats, whose brain weights fall between 1.1 and 2.4 grams and 
which were formerly used for the investigation on the cortical 
thickness (Sugita, 718 a), the area of the cortex was measured 
and the number of nerve cells, in a unit volume of 0.001 mm.* at 
a fixed locality of the cortex, was counted. These values were 
all later corrected to the corresponding values in the fresh con- 
dition of the material, using the correction-coefficients devised 
for this purpose. These results have been grouped and aver- 
aged according to the brain weight and then compared with the 
corresponding data in the Albino, which were presented in part I 
of this paper. 
2. The actual area of the cortex in the sagittal section may 
be obtained by the formula: L. F x T, x 1.20 (L. F and T,, in 
millimeters), where L. F is the longitudinal diameter of the 
cerebrum, 7’, is the thickness of the cortex in the sagittal sec- 
tion and 1.20 is a constant coefficient which was empirically 
determined (table 11, column G). 
3. The actual area of the cortex in the frontal section may be 
obtained, though less precisely, by the formula: W. D x T,, x 0.97 
(W. D and T,, in millimeters), where W. D is the frontal diameter 
of the cerebrum, 7’, is the thickness of the cortex in the frontal 
section and 0.97 is a constant coefficient which was determined 
empirically (table 12, column G). 
4. The percentage of the cortical area to the area of the whole 
frontal section is highest (48 per cent) in brains weighing 1.1 to 
1.8 grams. In a fully mature brain it has fallen to 44 per cent. 
5. The computed value for the volume of the entire cortex, 
mcdicated by theformula:b. F x W.DxT xC df, W.D 
and 7, in millimeters), where L. F is the longitudinal diameter, 
W. D is the frontal diameter of the cerebrum, 7’ is the average 
thickness of the cortex in the two sections and C a theoretically 
determined coefficient necessary to make the values directly 
comparable with the corresponding values for the albino rat, 
shows that the cortex is increasing relatively rapidly in the 
