116 NAOKI SUGITA 
Norway brains weighing less than 1.483 grams. After that stage 
its increase nearly keeps pace with the increase in the volume 
of the entire cerebrum. 
6. In Norway brains weighing from 1.1 to 2.2 grams, the cell 
density or the number of nerve cells in a unit volume of the 
lamina pyramidalis and the lamina ganglionaris, in a fixed lo- 
cality of the cortex, decreases slowly but steadily as the brain 
weight advances. It has proved slightly less than that in the 
Albino (compare table 16, column D, with table 7, column C). 
In the lamina ganglionaris the number of ganglion cells only in a 
unit volume is at its highest in the brains weighing 1.7-1.8 grams 
(table 14). 
7. The value for the computed number of nerve cells in the 
entire Norway cortex, indicated by the formula: N x L. F x 
W.DxTxC (L. F, W. D and T, in millimeters), where N 
is the number of cells in two unit volumes andl. F x W.D x 
T x C is the computed volume of the cortex, shows that it is 
almost completed in a brain weighing something more than — .37 
grams. 
8. Comparisons in respect of the above characters between 
the Norway and the Albino brains of the like weight show that, 
in the cortical areas in the sagittal and the frontal sections and 
in the volume of the entire cortex, the Norway rat surpasses the 
albino rat, but the number of cells as computed for the entire 
cortex may be regarded as the same in both forms. We conclude 
therefore that the difference in absolute brain weight between the 
two forms is not correlated with a difference in the number of 
nerve cells in the cerebral cortex. In a Norway brain weighing 
1.4 to 1.5 grams, which corresponds to an Albino brain weighing 
1.17 grams and is about twenty days in age, the elemental or- 
ganization of the cerebral cortex in the Norway rat is considered 
to be almost completed. 
