GROWTH OF THE CEREBRAL CORTEX 30 
body in the Albino is about 20 to 40 per cent less than in the 
Norway rat of like age (table 9). Accordingly, the albino brain 
should be about 18 per cent or more, less than the Norway brain 
of like age, and the data for the thickness of the cortex in the 
two forms show a fairly constant relation, when plotted as in 
chart 9 in accordance with this assumption (see also table 3, 
Sugita, 717 a). 
As stated, Norway rats under about 10 days of age have not 
been studied, but a comparison of the graph for the thickness of 
the cortex in the normal albino rat with the graph forthe Norway 
cortex displaced for age makes it reasonable to assume that a 
Norway brain which weighs 1.16 grams (Group N XI) corre- 
sponds to an albino brain which weighs about 0.95 grams (Group 
IX), at which stage the cerebral cortices of the both forms have 
nearly completed their active growth in thickness and are going 
over to the second phase, during which the cortical area keeps 
pace with the increase in brain volume. It may be assumed 
also (see later) that, in the Norway rat, with a brain weight of 
about 1.4 grams the cortical myelination is beginning to take 
place. 
Thus in the postnatal life of the Norway rat, the first phase of 
the development of the cerebral cortex covers the period during 
which the brain weight increases to 1.16 grams from birth, when 
the brain weight is about 0.25 grams, and the second phase of 
the cortical development covers the period, during which the 
brain weight increases from 1.16 grams to about 1.44 (Group 
N XIV) when the cortex attains within 4 per cent the full thick- 
ness. By the middle of the second phase the process of myelina- 
tion is active, and before the end of this phase the cortex has 
already attained nearly its full thickness. 
This assumption, that the completion of the cortical develop- 
ment in thickness coincides with the period of active myelination, 
is supported ‘by another set of facts. Table 10 gives the abso- 
lute weights of the dry substance in the brain of the Norway rat, 
arranged according to brain weight. These values were calcu- 
lated by me from tables originally given by Donaldson and 
Hatai (11). The data are plotted in chart 10, which also gives 
the corresponding data for the albino rat, in a dotted curve. 
