82 NAOKI SUGITA 
transitional layers, which will be replaced by the ecapsula ex- 
terna, are relatively wide during this phase. 
After twenty days, when the brain has attained nearly the 
weight of 1.17 grams, the remainder of the section (the central 
nuclei and the white substance) increases more rapidly than 
the cortical area and the group of proliferating cells in the ven- 
tricular wall disappears. The percentage of the cortical area 
to that of the whole section consequently decreases under these 
conditions, though the absolute value of the cortical area is 
steadily increasing. 
From the mode of the changes in the percentage value of the 
cortical area, we may conclude that, in the albino rat, at least, 
the period during which the brain weight increases from 0.25 
gram (birth) to 1.2 grams (about 20 days), is the period when 
the cortical elements are principally produced, matured and 
arranged, and that the cortex is precocious in its construction. 
The growth or construction of the remaining parts in the frontal 
section, so far at least as this is expressed by increase in volume, 
is relatively retarded or delayed until the cortex has acquired 
all its characteristic elements. 
H. The volume of the entire cortex 
The true volume of the entire cerebral cortex can not be 
measured by the methods here used. It will require a special 
study for that purpose. My present object is to obtain relative 
values for the cortical volume and a record of the change in 
these relative values according to brain growth. If the data 
for the area of the cerebral cortex as measured by me in the two 
typical sections be reduced to a simple geometrical form, it will 
be very easy to compare the changes in the computed volume in 
successive brain weight groups. As already mentioned, the 
cortical area in the sagittal and the frontal sections, which 
sections cross one another at right angles, may be reduced to 
rectangles which have as the long side the lengths proportional 
respectively to the sagittal and the frontal diameters of the 
cerebrum (L.F and W.D), and as the short side the cortical 
