GROWTH OF A TRANSPLANTABLE TUMOR IN MICE 325 
of the factors of this complex encouraging temporary growth, 
have given a growth percentage of 17.54+0.83 in 969 observa- 
tions. This differs from the 11.12 per cent of the non-susceptible 
common race (N) by 6.7 times the probable error. 
6. The percentage of growth in the back-cross (B.C.) race 
decreases for the second and third weeks after inoculation, but 
reaches and holds a level at about 13 per cent, where it remains 
during the fourth, fifth, and sixth weeks. 
7. Mice of this race when inoculated at from two to ten days 
old show a lower percentage of growth than mice from twelve 
to twenty plus days old. The difference is 4.6 times its probable 
error and the percentage 13.77+1.05 and 21.58+1.28, respec- 
tively. 
8. The difference between the age groups is chiefly confined to 
the female sex. The difference between males of the upper and 
lower groups is less than its probable error. The difference be- 
tween females of the upper and lower group is five times its prob- 
able error, the percentages being 25.74+2.07 and 12.12+1.76, 
respectively. 
9. Females of the upper age group in both series are, during the 
later periods of observation, at an age when sexual maturity is 
attained. Sexualmaturity by the awakening activity of the ovary 
means further differentiation and further development of indi- 
viduality of the tissue. Such assumption of individuality leads 
to elimination of the tumor in non-susceptible animals [series 
(N)] and to encouragement of its growth in animals inheriting 
all or part of the factors which are contributed by, and which 
characterize susceptible animals of the Japanese waltzing race 
in which the tumor originated. 
10. The factors underlying susceptibility to the implanted 
tumor J.W.B. are inherited units introduced into the back-cross 
(B.C.) generation by the Japanese waltzing grandparent through 
the F,; hybrid parent. 
11. Certain of these factors, therefore, find their active ex- 
pression as favorable agents in supporting growth of the tumor 
implants in female animals, at the onset of sexual maturity. 
This means that any attempt to analyze the biological nature of 
