GROWTH OF THE CEREBRAL CORTEX 91 
in the sections of the newborn brains, many immature cells, 
the indifferent cells and the neuroblasts, crowded densely to- 
gether in the ventricular wall and in the transitional layers 
and these are all migrating to the cortex. The number of cells 
in the cortex is increased after birth by receiving these cells 
already formed but lying at birth still outside of the cortex proper, 
besides by receiving the cells which are newly formed after birth. 
So the nerve cells, destined for the cortex, are largely present in 
immature form in the cerebrum at birth, but lie outside of the 
cortex proper, while the actual number of cells formed after 
birth may amount only to a small fraction of the total number 
of nerve cells in the cortex at maturity, though there is active 
mitosis during the first days after birth. 
I have previously recognized three developmental phases in 
the growth of the cortex in thickness (Sugita, ’17 a), as follows: 
First phase, from birth to the 10th day. 
Second phase, from the 10th to the 20th day. 
Third phase, from the 20th to the 90th day. 
The first and the second phases here given may also be applied, 
without any modification, to the changes in cell number in the 
cerebral cortex, while the third phase does not appear in this 
connection. 
IV. CONCLUSIONS 
In an earlier study on the cerebral cortex of the albino rat 
(Sugita, ’17 a), I stated that the cerebral cortex attains nearly 
its full thickness at the age of twenty days, before myelination 
in the cortex had begun, and that the organization of the cerebral 
cortex might be considered as precocious, having been provided 
with all its mechanisms at the time of weaning. At this age, 
the brain weight is only a little more than one-half the weight 
at maturity. Size, volume and weight of the entire brain are 
all midway in their growth, but there have appeared no striking 
changes by which we might guess from the gross appearance of 
the brain anything about the numerical completeness of its 
cortical elements. Just at this stage, however, cell division in 
the cerebrum has almost ceased (Allen, 712). 
