GROWTH OF THE CEREBRAL CORTEX 37 
This chart shows clearly that the solids in the Norway brain 
increase rapidly after the brain weight has reached something 
more than 1.2 grams (see x). This turning point of the graph 
corresponds to 0.95 grams of brain weight in the Albino (see *). 
It was found in the albino rat that, when the brain weight has 
surpassed 1.15 grams, namely 0.95 plus 0.20 grams, myelination 
of the cortical fibers is active. Hence, in the Norway brain, the 
myelination in the cortex should be active when the brain weight 
has reached 1.44 grams, namely somewhat more than 1.2 plus 
0.2 grams. Furthermore, as we have seen that in the albino rat 
the beginning of myelination in the cortex coincides with the 
phase when the cortex has nearly attained its full thickness, so 
we see the same relations in the Norway rat also. 
From these facts we conclude that the brains of the both forms 
pass through the same course of cortical development according to 
age, as the span of life is the same in the two. The weights of 
the brains which are in the same stage of development, are how- 
ever not the same in the both forms, being in the Norway rat 
about 18 per cent—the Norway brain weight being taken as 
the standard—heavier than in the albino rat. The statement of 
Donaldson which was expressed in the paper cited, to wit: “‘If 
in the animals compared the brain weights are the same, then 
the Norway rat has a smaller body weight and a higher percent- 
age of water in the central nervous system,” might be rewritten 
as follows: When ages are the same, the Norway rat has a greater 
body weight, a heavier brain (18 per cent more in weight), a 
thicker cortex and nearly the same percentage of water in the 
central nervous system. 
A comparison of the cortical development in the two forms 
can be made adequately only by first reducing by 18 per cent 
the actual brain weight of the Norway rat and then comparing 
the cortex in both forms according to the corrected brain weight. 
Since mature Norway brains have only a slightly greater volume 
than the Albino brains of like weight (see table 4 A, Sugita, ’18), 
but at the same time have a cerebral cortex on the average 8 per 
cent thicker, it follows that in the Norway brain the proportion 
of gray substance is greater. This difference apparently accounts 
for the higher percentage of water found in the Norway brain. 
