114 NAOKI SUGITA 
IX. CONCLUSIONS 
Putting together the foregoing observations, we come to the 
conclusion that in the case of the Norway rat brain the entire 
volume of the cerebral cortex is actively increasing up to a 
brain weight of something more than 1.43 grams (Group N XIV) 
and that the number of nerve cells in the cortex is completed in 
a brain weighing something less than 1.43 grams (Group N XIV) 
(chart 4). After this, the increase in cortical volume keeps 
pace with the enlargement of the entire cerebrum, showing that 
the cortical mass and the remainder of the cerebrum are growing 
at the same rate. So, the end of the short period during which 
the brain has attained 1.37 to 1.54 grams in weight (Groups 
N XIII to N XV) marks an epoch in' the development of the 
cerebral cortex of the Norway rat, at which the structural com- 
pletion of the cortex has been acquired and the full prepara- 
tion for the functional education has been established. This 
period corresponds approximately to the age of twenty days. 
In the Albino, the same degree of development is ‘reached 
when the brain attains a weight of 1.17 grams or is twenty days 
old. As I suggested in an earlier paper (Sugita, ’18 a), a Nor- 
way brain corresponds in the development of the cortex to an 
Albino brain weighing about 18 per cent: less. This assumption 
has held true in the present examinations of the cortical volume 
and cell number, because an Albino brain weighing 1.17 grams 
just corresponds to a Norway brain weighing 1.43 grams. 
The number of cells in the Norway cortex has been shown to 
be but slightly (1 per cent) different from that in the Albino rat 
cortex and may be regarded as the same in both forms. This 
fact justifies at the same time a conclusion reached by Donald- 
son in his former comparison of the Norway with the Albino rats, 
that the greater weight of the brain in the Norway rat, compared 
with the Albino of the same body weight or of the same age, is 
probably due to an enlargement of the constituent neurons rather 
than to an increase in their number (Donaldson and Hatai, ’11). 
The results of my study regarding the cell size in the cortex in 
these two forms will be discussed in a forthcoming paper and 
will support the statement just made. 
