GROWTH OF THE CEREBRAI:. CORTEX 93 
V. SUMMARY 
1. Employing the sagittal and the frontal sections of 78 
albino rats, which were formerly used for the investigation on 
the thickness of the cerebral cortex (Sugita, 717 a), I made 
further measurements on the area of the cortex in these sections 
and counted at a fixed locality the number of nerve cells contained 
in a unit volume of 0.001 mm.,? in brains from birth to maturity. 
2. The observed data were all corrected to the values for the 
fresh condition of the material, by the use of the correction- 
coefficients based on the observations. The results were grouped 
and averaged according to the brain weight groups and the 
postnatal growth changes systematically analysed. 
3. The area of the cortex shown in the sagittal section is found 
to be proportional to the value L. F x T,, where L. F is the 
longitudinal diameter of the cerebrum and T’, is the average 
thickness of the cerebral cortex in the sagittal section. The 
actual area (after five days of age) may be calculated by the 
formulas H.6F <x 22 Ch. F and T.;. im. mullumeters)): 
where 1.22 is a constant coefficient which was empirically de- 
termined (see table 4, column G). 
4. The area of the cortex shown in the frontal section (of one 
hemicerebrum) is found to be proportional to the value W.D x T,,, 
where W. D isthe frontal diameter of the cerebrum and T, is 
the average thickness of the cortex in the frontal section. The 
actual area (after five days of age) may be calculated by the 
formula: W.D x T, X 0.91 (W.D and T,, in millimeters), 
where 0.91 is a constant coefficient which was empirically de- 
termined (table 5, column G). 
5. The percentage of the total’ area of that frontal section 
which is represented by the cortical area is least at birth (34 
per cent) and increases as the age advances till it reaches the 
maximum (50 per cent) at the period of 7 to 20 days (brain 
weight 0.75 to 1.25 grams). It then decreases, slowly and at 
maturity is less than 44 per cent (table 2). This means that 
during the first 7 days the cortex is increasing in area more 
rapidly than the remainder of the section, while during the 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 29, NO. 2 
