318 CC. LITTLE 
not greatly different from the percentage found in all male 
lines and in the total percentages of females in the (N) series. 
Instead of showing a falling off of growth as in the (N) series, 
the females of the (B.C.) series comprising the upper age group 
give a percentage of 25.74+2.07. This group differs from the 
lower group of the same series by 13.62+2.71, a difference of 
five times its probable error. 
To sum up for discussion the main facts made apparent by the 
experiments, it may be stated that: 
1. In the common-stock mice of series (N), there is a steady 
decrease in the percentage of observations showing growth on 
successive weeks after inoculation. In the back-cross hybrids of 
series (B.C.), the percentage falls during the second, third, and 
fourth weeks after inoculation, but comes to a resting point at 
about 13 per cent for the fifth and sixth weeks. 
2. In series (N), the percentage of growth in males and in 
females of all ages grouped is extremely close, 13.17 and 13.88, 
respectively. The males show no significant difference between 
upper and lower age groups. The females show in the lower 
group a percentage of 19.46 and in the upper group 9.39. The 
difference between these groups is seven times its probable 
error. 
5. In series (B.C.), there is no significant difference between 
the total percentages for males and for females. The two age 
groups for males are also very similar in growth percentage. 
The females, however, show a great increase of growth in the 
upper age group, 25.74 as compared with 12.12. This difference 
is five times its probable error. 
DISCUSSION 
The common stock used in series (N) was one in which we 
should expect that few if any of the animals inoculated would 
show a steady progressive growth of the Japanese mouse tumor. 
Upon testing, it had been shown repeatedly that this race was not 
like the Japanese waltzing mouse in size, color, vigor, or indeed 
in any visible characters. It is not therefore surprising to find 
a negligible occurrence of permanent growth and a small amount 
