86 NAOKI SUGITA 
slower and is somewhat less in rate than the increase in cere- 
bral volume or brain weight, as is also seen in the graphs given 
in chart 2. ; 
I. Number of cells in a unit volume of the cortex 
In the lamina pyramidalis of the newborn Albino brain, at a 
dorso-lateral part of the pallium, where, in the frontal section, 
the cell count was nYade (fig. 2, VII), there were in the fresh 
condition about 500 pyramids crowded in a unit volume of 0.001 
mm.’ This number decreases, as the brain grows, and falls to 
110 in a brain weighing about 1.2 grams (20 days) (table 3). 
In a brain weighing about 1.5 grams (50 days), the number has 
dropped nearly to 90, from which it is only slightly reduced in 
the heavier brains. In an old rat, whose brain weighs more 
than 2.0 grams, the number is about 80, or less than one-sixth 
the number at birth. According to another study, which will 
be published later, the size of the cell body and of the nucleus 
of the pyramids in the lamina pyramidalis, measured at this 
same locality, increases very rapidly during the first ten days 
after birth, till the brain has attained 0.9 gram in weight, when 
these structures reach their maximum size (Cell body 16u X 20u; 
Nucleus 14u X 15y). After this stage the cell body and the 
nucleus are mature in their nucleus-plasma relation, but still 
changing their chemical composition, as revealed by the stains, 
while the neuron as a whole is still growing as shown by the 
developing axon and the dendrites.’ This fact is in accord with 
the observation that the number of pyramidal cells in the lamina 
pyramidalis decreases rapidly after birth, until the brain weight 
reaches about 0.9 gram (10 days), after which the rate of decrease 
becomes slow. 
The change in cell number in a given volume of the cortex 
during the growth of the brain is determined by two main 
factors: (1) the enlargement of the cell body proper and the 
growth of the cell branches and (2) the development of the 
intercellular structures (that is, incoming nerve fibers, neuroglia, 
blood vessels) and myelin formation, separating the cells more 
and more from each other. 
