94 NAOKI SUGITA 
following 13 days its rate of increase is similar to that of the re- 
mainder. After 20 days the rate of increase for the remainder 
surpasses that for the cortex. 
6. The actual volume of the cortex could not be obtained by 
the use of the data now available, but the computed volumes 
of the cerebral cortex at different ages (comparable among 
themselves), may be found by the use of the formula: L. F x 
W.D x T (all in millimeters), where L. F is the longitudinal 
diameter of the cerebrum, W.D is the frontal diameter of the 
cerebrum and 7’ is the average thickness of the cortex. The 
volume increases most rapidly during the first ten days after 
birth and the rate of increase in the cortical volume continues to 
surpass the rate of increase in the entire cerebral volume, dur- 
ing the first twenty days. After twenty days the cortex increases 
at a somewhat lower rate than the increase of the entire cere- 
brum in volume (chart 2). 
7. The number of cells contained in a unit volume of 0.001 
mm.’ of the cortex indicates the cell density of the locality where 
the count was made. In the lamina pyramidalis the pyramids 
are most crowded at birth and the number in the unit volume 
decreases rapidly during the first ten days after birth. After 
twenty days it decreases slowly but steadily, the number at 
maturity being about one-sixth the number at birth. As for 
the lamina ganglionaris, the total cell number in the unit volume 
(the small and the large pyramids taken together), is at its 
highest value at birth. It decreases relatively rapidly during 
the first twenty-five days after birth, then is slightly increased 
for a time, after which it decreases again slowly and at full 
maturity it shows about one-fifth the number present at birth. 
Taking the large ganglion cells alone, we find that the number 
decreases rapidly during the first eight to ten days, then remains 
the same up to twenty days, after which it decreases again, 
showing two-sevenths the initial number at full maturity. The 
decrease in cell-density according to brain growth is due to the 
enlargement of cell bodies, the development of cell attachments, 
the separation of cells from each other through myelination, 
ingrowing fibers and other changes. The average cell density, 
