242 NAOKI SUGITA 
to some similar studies by other authors. These data give us a 
general idea of the postnatal development of the cerebral cortex 
in a representative mammal (albino rat), and we may fairly 
infer that similar changes occur in other mammals during the 
growth of the brain. To test how far my conclusions on the 
mode of the development of the cerebral elements during post- 
natal life may be extended, I shall review and summarize in the 
present paper the results obtained by several authors on the 
development of the cortex in other mammals and make a com- 
parison of their results with the data obtained by me. 
II. THICKNESS OF THE CEREBRAL CORTEX IN THE ALBINO RAT 
The results obtained by me regarding the cortical thickness 
in the brain of the albino rat may be summar zed as follows 
(Sugita, 717 a): 
1. The cortex at the frontal pole of the hemisphere is the 
thickest and that at the occipital pole is the thinnest. Speaking 
in general terms, the cortex diminishes in thickness from the 
frontal to the occipital pole and from the dorsal to the ventral 
aspect. 
2. After birth, the general average of the cortical thickness 
increases very rapidly during the first ten days, thickening from 
0.74 mm. at birth to 1.73 mm. at ten days, more than twice the 
thickness at birth, while the brain weight increases from 0.25 
gram to 0.95 gram during the same period. ‘This is designated 
by me the first phase of the cortical development. 
3. Between the tenth and the twentieth day after birth, the 
cortical thickness increases more slowly, attaining at twenty 
days to within 4 per cent of the full thickness of the cortex, 
namely, 1.84 mm., or about 2.5 times the thickness at birth, 
while the brain weight increases to 1.15 grams. This is desig- 
nated the second phase of the cortical development. 
4. From the twentieth to the ninetieth day, the cortical thick- 
ness increases but little on the average, attaining at ninety days 
the thickness of 1.93 mm., or 2.6 times the thickness at birth, 
while the brain weight has increased to about 1.80 grams. This 
is designated the third phase of the cortical development. 
