96 NAOKI SUGITA 
weaning time of the rat... If the cerebral organization of the rat 
brain at five days of age is similar to that of the man at birth, 
and the growth processes in the rat are thirty times as rapid as 
in man, then the completion of the cortex which occurs in the 
rat brain at twenty days should occur in the human brain at 
about fifteenth month of age. 
PART 
ON THE AREA OF THE, CORTEX AND ON THE NUMBER OF CELLS 
IN A UNIT VOLUME, MEASURED ON THE FRONTAL AND SAGITTAL 
SECTIONS OF THE BRAIN OF THE NORWAY RAT (mus NOR- 
VEGICUS), AND COMPARED WITH THE CORRESPONDING DATA FOR 
THE ALBINO RAT 
VI. INTRODUCTION 
In the Part I of this paper, I have presented the data on the 
area of the cerebral cortex measured on the sagittal and the 
frontal sections of the Albino rat brain and on the number of 
nerve cells in a unit volume of the cerebral cortex, and, by cal- 
culations based on these data, I have come to the conclusion 
that the entire volume of the cerebral cortex is increasing most 
rapidly during the first ten days after birth, while from twenty 
days onwards it increases at a lower rate than the entire cerebral 
volume. Further, the computed number of nerve cells in the 
entire cerebral cortex also increases very rapidly during the first 
ten days after birth and attains nearly its complete number at 
the age of twenty days. 
I now wish to compare these relations in the Albino with 
those in the Norway rat, in the same manner as I have already 
done in the matter of the growth of the brain in size (Sugita, 
18) and of the thickness of the cortex (Sugita, ’18 a). 
Employing for the Norway brains the sections on which the 
cortical thickness was measured earlier and for which the in- 
dividual body measurements have been already given in table 
1 in my fourth paper (Sugita, ’18 a), I have measured the area 
