GROWTH OF A TRANSPLANTABLE TUMOR IN MICE 319 
of temporary growth diminishing on the successive weeks of 
observation and finally disappearing altogether. 
If we suppose that in this race temporary growth of the 
Japanese waltzing mouse tumor was possible in certain of the 
very young mice until an intolerance toward it was developed 
by the normal maturing and differentiation of the tissues of 
the non-susceptible stock animal, a working hypothesis is 
provided. 
We know from the work of Murphy and others that embryonic 
tissue provides a suitable growth medium for tissue of even 
widely divergent forms. The idea suggested is that such em- 
bryonic tissue provides a medium favorable to such growth. 
It seems, however, that tolerant is a more accurate description 
than favorable. The relatively undifferentiated characterless 
embryonic tissue is not possessed of sufficient biological indi- 
viduality to discriminate against and set up unfavorable reac- 
tions toward the implant of foreign tissue. Not until it reaches 
a sufficiently advanced stage of differentiation do we find an 
unfavorable reaction initiated resulting in the final elimination of 
the implant. 
If such an explanation is correct and applicable to the case of 
mice, we should expect to find, as we do, the younger and rela- 
tively less differentiated mice more tolerant of the foreign im- 
plants, and therefore showing a higher percentage of growth than 
do the mice which are older and more differentiated at inocula- 
tion. In this connection the difference between the sexes is of 
ereat interest. It is a well-known fact that female mice nor- 
mally come to sexual maturity at an earlier age than do their 
litter brothers. It follows, therefore, since sexual maturity im- 
ples differentiation, that the female mice would reach earlier 
than the males a degree of individuality, which would enable 
them to discriminate more strongly against the foreign tissue. 
This is shown in the striking decrease in the percentage of growth 
in the females of the older age group as compared with those of 
the younger age group. These animals have, in all probability, 
attained a sufficient degree of development of individuality to 
recognize the tumor implant as a foreign body and to therefore 
