92 NAOKI SUGITA 
From the data now available, I conclude that, in the albino 
rat, the cerebral cortex exhibits the complete number of nerve 
cells at about the 20th day after birth, at which age some of the 
cells have attained their full size. The area of the cortex in the 
sagittal and in the frontal sections has shown a continuous 
increase throughout life and no radical change in rate occurs at 
this time. But, if the thickness of the cortex be taken into 
consideration and the volume of the entire cortex be calculated, 
it becomes clear that the entire volume of the cerebral cortex 
has stopped the more active increase, which it made earlier, 
at about the 20th day, and after that the increase becomes 
slow, and lower in rate than the increase in the volume of the 
cerebrum. If, further, the cell density in the cortex be con- 
sidered, it is found that the number of cells as computed for 
the cerebral cortex (table 7, column E) increases rapidly, 
especially during the first ten days after birth, but exhibits 
nearly its complete number at the age of twenty days, after 
which it shows no significant change. 
We may conclude therefore that the cerebral cortex has been 
completely organized at the age of about twenty days, and that 
the further development of the cortex does not involve:an in- 
crease in cell number, but involves mainly the maturing of ele- 
ments already provided. The education of the cerebral cortex 
as a whole might properly be said to begin after this age, the 
preceding period having been largely one of preparation or 
construction. It is of interest to note that this epoch cor- 
responds to the weaning time of the rat. 
According to the study of Donaldson (’08) on the comparison 
of the albino rat and man, the rat grows thirty times as fast as 
man. When however the brain of the rat is to be compared 
with that of man, it must be remembered that at birth the human 
brain is somewhat more mature and corresponds in organiza- 
tion not with the rat brain at birth but at five days of age (Don- 
aldson MS8.). This being the case, the rat cortex at the 20th 
day of postnatal life probably corresponds with the human 
cortex at the 15th month (20 less 5). This conclusion has not 
yet been tested. 7 
