GROWTH OF THE CEREBRAL CORTEX 95 
represented by the sum of the numbers in the lamina pyra- 
midalis and the lamina gariglionaris, given as N in table 7, de- 
creases rapidly during the first ten days and after that the de- 
crease becomes very slow and steady, showing at maturity a 
density of about one-fifth of that at birth. 
8. The computed value for the number of cells in the entire 
cerebral cortex may be determined by the formula: NV x L. F Xx 
W.D xT (L.F,W.D and T, all in millimeters), where L. F 
is the longitudinal diameter of the cerebrum, W.D the frontal 
diameter of the cerebrum, 7 the average thickness of the sa- 
gittal and frontal cortex and N the average number of the 
nerve cells in two unit volumes of the cortex, at the particular 
locality (locality VII) where the counts were made. This com- 
puted value for the number of nerve cells in the entire cerebral 
cortex increases rapidly during the first ten days, at the end of 
which period it attains nearly 1.9 times the value at birth. 
During the following ten days, it increases slowly but steadily, 
and it attains its complete number at the age of twenty days 
(brain weight 1.17 grams). After this age the number of nerve 
cells is almost constant. The number of cells at maturity is 
twice the number at birth. 
It is recognized that this conclusion concerning the number 
of nerve cells in the cortex at various ages is based on enumera- 
tions in only two cortical layers at but one locality, and that 
on this ground its general value might be questioned. When 
it is recalled however that table 11 in a preceding study on the 
growth of the cortex in thickness (Sugita, 717 a) shows all the 
localities measured in the cortex to undergo the same relative 
increase in thickness between birth and maturity, and always 
to stand in the same relation to one another, the doubts with 
regard to the general value of these particular results are largely 
removed. 
9. Considered all together, the data on the development of 
the cerebral cortex indicate that it has been completely organ- 
ized in the albino rat at the age of twenty days. The further 
development after this age represents a maturing of the elements. 
The completion of the cerebral organization corresponds to the 
