122 LEONELL c. STRONG 
expanded the experiment to include the whole life of the individ- 
ual. The five age groups were selected so as to include individ- 
uals from the five important periods of the life (infancy, puberty, 
adolescence, maturity, and senescence). The interesting point 
discovered in our experiments is that susceptibility to trans- 
plantable tumors increases with old age. The age susceptibility 
toward transplantable tumors (for a non-susceptible race) has 
a remarkable relation to the activity of the gonads (fig. 7, 
p. 89). Susceptibility decreases at first rapidly, then gradually, 
from birth through the period of puberty and adolescence up 
to the beginning of gonad activity (maturity). The minimum 
percentage of growth reactions are obtained with mice one-half 
to three-fourths grown. 
Susceptibility remains fairly uniform throughout the period 
of maturity. With senescence, however, and the accompany- 
ing decreased activity of the gonads, susceptibility rapidly in- 
creases. This increase in susceptibility in old mice is mainly 
confined to males and will be discussed under the heading of 
sex. The same explanation offered by Little for the first part 
of the life curve can be applied in our own case. Susceptibility 
to transplantable tumors depends considerably upon the degree 
in which the tissues of the host have attained a physiological 
specificity, when fully developed will characterize the particular 
individual in question. The most characteristic physiological 
activity of the individual is attained at the period of maturity. 
With maturity the animal manifests its most pronounced bio- 
logical characteristics. The susceptibility to transplantable 
tumors, being a complex mendelian phenomenon, can only be 
tested by using mature individuals. 
The susceptibility curve suggested the possibility of analyzing 
the factors which underlie tissue specificity more carefully by 
removing the gonads from individuals in all the different age 
groups before inoculating them with the transplantable tumor. 
This experiment will be discussed after the section on the sex 
factor. 
