328 CHI PING 
weight of 346 grams. As compared with ratios for standard 
Albinos of like body weights (table 2, Ping ’21), the ratios 
for the Norway are clearly low. 
DISCUSSION 
In the foregoing paragraphs the growth of the cells in the 
Norway rat has been treated in relation to the body weight 
and length and in relation to age, either observed or inferred 
from the percentage of water in the brain of the animal. In 
each case the results show that the growth is comparatively 
rapid at first and then becomes gradual. Moreover, as was to 
be expected, the growth of the sympathetic nerve cells of the 
Norway resembles in a general way that of the standard Albino. 
There are, however, differences between these forms, worthy 
of note, and to make the comparison as complete as possible, 
the data for the inbreds will also be taken into consideration. 
Although the form of these data and the numbers of cases 
are not the same in the three series, they are yet sufficiently 
similar to make several comparisons worth while. 
Before attempting this, a word about the general relations 
of the three strains here examined is in place. Both the Albino 
strains contained animals which had been in captivity for many 
generations, and were also domesticated in the sense that they 
had lost the fear of man and were easy to handle. The Norway 
strain, on the other hand, had two groups in it: (1) the rats 
caught wild and of unknown age—represented in this series by 
animals 37 grams or more in body weight-—and (2) a group 
which were the F, or F; descendants of Norway parents caught 
wild. Although this second group was composed of captive 
individuals, they were by no means domesticated and for the 
most part were still timid and excitable. 
The differences between these strains may be tabulated as 
follows: 
Albinos Norways 
Not pigmented Pigmented 
Captive Wild or captive 
Domesticated Not domesticated 
(A) ‘Standard’ (not inbred) Not inbred 
(B) ‘Inbred’ 
