BRAIN WEIGHTS OF RATS Bad 
The frequency distribution of the brain weights within the 
litter seems to be continuous, and thus the Norway type or 
albino type cannot readily be differentiated. Consequently I 
have adopted the statistical method of taking the averages of 
all the rats belonging to each generation separately. At the 
same time I have plotted all individual brain weights in chart J 
in order to show the extent of the variation, as well as the rel- 
ative positions held by the records belonging to the different 
generations. 
The averages were taken for the sexes combined. This pro- 
cedure affords some advantage since by this means we can deal 
with a larger number of rats and thus can eliminate irregularities 
arising from the averages of smaller data. Any disturbance 
which arises from the sexual difference in the brain weights is 
almost insignificant since such a difference amounts to less 
than 0.5 per cent in the albinos and about 1 per cent in the 
Norway rats. The formulas given above may be applied 
directly when the sexes are not distinguished. 
BRAIN WEIGHTS IN THE NORWAY AND ALBINO RATS 
In table I are given the data on the brain weights for both 
the norway and albino rats which have been used as parents of 
I’, in the present experiment. As will be seen some rats are 
recorded in the table several times. This repetition was ne- 
cessitated by the fact that these rats were crossed several times 
with different rats and thus they were counted as often as the 
matings were made. 
Norway rat. The observed average brain weight of the Nor- 
way rats with a body length of 237 mm. is 2.251 grams. Ac- 
cording to the formula the Norway rat with a body length of 237 
mm. should give a brain weight of 2.295 grams. Thus the 
observed brain weight is 1.91 per cent less than the value given 
by the formula. The computed brain weights for the Norways 
which appear in chart I were obtained by subtracting 1.91 per 
cent from the formula values. 
