GROWTH OF THE CEREBRAL CORTEX j Zoo 
the absolute mass of the extractive substances is 0.065 gram 
(about 5 per cent of the brain weight) less than in the controls 
of the same age. The absolute weights were in the underfed 
0.626 gram and in the controls 0.691 gram. These extractives 
represent mainly the myelin which is contained in the sheaths of 
the nerve fibers, and the above results mean that the extractive 
substances are increasing at the same rate or slightly slower in 
the underfed than in the controls. 
In my material as seen in table 14, the underfed brains contain 
slightly more water (by 0.48 per cent on the average) than the 
standard of like age, and, as presented above (chapter 13), the 
frontal section showed a higher percentage in area of the cortex 
against the area of the central nuclei, which latter contain the 
bulk of the myelin sheaths. On the other hand, the underfed 
brains contain less water (by 1.4 per cent on the average) than 
the standards of the same brain weight. Comparisons of the 
absolute weight of the solids in the underfed brain with the corre- 
sponding standard value for the same brain weight and for the 
same age, based on data in ‘The Rat’ (Donaldson, 15), are given 
‘in table 18. The underfed has shown as a rule considerably less 
in total solids than the standard for the same age, though it 
proved to be only slightly higher in percentage of water than the 
standard (also table 14). 
From the above, the absolute mass of the solids in the underfed 
brain seems to be more than in the standard for the same brain 
weight, but less than in the standard for the same age. So the 
increase in solid mass is somewhat retarded by starvation. It will 
be noted that in my series of rats the percentage of water in the 
underfed was 0.48 per cent above that in the standards of like 
age and this is the reverse of the results reported by Donaldson 
and by Hatai in the studies just cited. This discrepancy prob- 
ably depends on the fact that my rats are absolutely much 
younger than those studied by the other authors, but the ex- 
planation must await further study. 
Since, as shown in chart 1, the relative values of the quantity 
of the alcohol-extractives has a fixed relation to the absolute 
weight of the solids in the brain, the above statement may be also 
