SHINKISHI HATAI 
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MATERIAL 
The Norway rats here used were trapped at different points 
in the city of Philadelphia, while the Albinos were secured from 
the colony kept at The Wistar Institute. The total number 
of the Norway rats employed was 6 males and 4 females, and of 
the Albinos used, 3 were males and 9 females. With these rats 
14 matings were made successfully. Although more than twice 
the number of rats named above were mated at the beginning 
of the experiment, the data for many had to be discarded on 
account either of the loss of parents or of the sterility of the = 
hybrids of the F, generation. Only those cases in which the 
data on the parents as well as their offspring for the first two 
generations were secured, were used in the present paper. The 
total number of the hybrids examined was 110 F; and 149 F». 
THE METHOD OF COMPARING THE BRAIN WEIGHTS 
In comparing the brain weights in the hybrids with those in 
the parents, the following method was adopted. As has -been 
already found by Donaldson (’08) with the albino rat, and by 
Donaldson and Hatai (’11) with the Norway rats, the brain 
weight varies with the body length according to a logarithmic 
law. Since the body lengths in the hybrids were not identical 
with those in the parents, a direct comparison of the observed 
brain weights without a due consideration of the difference in 
the body length was not justified. The proper adjustment was 
however made possible by utilizing the following formulas (for 
sexes combined). 
For Norway rat (Donaldson and Hatai, ’11). 
Body length + 165 
10 Hp8 — 18 
Brain weight = 0.825 log (Body weight — 4) +0.233 
For albino rat (Donaldson, ’08) 
Body length + 134 
Body weight = 10 i —15 
Brain weight = 0.569 log (Body weight — 8.7) + 0.554 
Body weight 
