84 HENRY H. DONALDSON 
Inspection of the data at hand suggests that size differences 
of the brain may depend either on a mere magnification or re- 
duction of the neurons in a strictly proportional manner or on a 
disproportional growth caused by a relative excess of white sub- 
stance, in the heavier brain and vice-versa. 
As will be pointed out further on, the least variability in the 
water content of the brain is found within the same litter, and 
it seems probable that here the differences in the brain size which 
occur are mainly due to a strictly proportional growth of the 
neurons. On the other hand, it appears that the brain which is 
large at a given age has commonly anticipated some of the 
growth changes whch belong to a later period, and this means 
that the relative abundance of the myelinated axons has been 
increased—a change necessarily accompanied by a lowering of 
the water content. This, the more common relation found be- 
tween two unrelated rat brains of the same sex at like age, 
where the larger brain usually has the smaller percentage of 
water, is considered therefore to be due to the relative excess 
of myelinated fiber substance in the larger brain. As will be 
seen, this statement constitutes a reversal of the opmion which 
I formerly held (Donaldson, 710). 
If this corrected opinion is accepted, the next step is to de- 
termine the factors to be used for reducing any observation on 
the percentage of water. 
On the relation of the percentage of water to the absolute brain 
weight 
According to our hypothesis the relatively small brain is 
likely to be retarded in development, i.e., to be a trifle behind 
the stage characteristic for its age and so to have a less pro- 
portion of myelin and therefore a higher percentage of water, 
while on the other hand the relatively large brain is likely to be 
precocious and to show as a consequence a lower percentage of 
water. 
The test of this assumption. was therefore made by averaging 
for the first and last thirds or halves of each age group, ar- 
