308 Cc. C. LITTLE 
race. The common mice used are designated as series (N), the 
back-cross hybrids as series (B.C.), and their relationship and 
derivation is shown in the following diagram. 
It will be seen at a glance that the two series are very differ- 
ent biologically. Series N includes common-stock mice unre- 
lated to Japanese waltzing mice (the race in which the tumor 
originated and in which it grows freely). Common mice rarely, 
if ever, have shown progressive uninterrupted growth of the 
Japanese tumor J.W.B., although, as Tyzzer and the writer 
(16) have shown, there may be temporary growth of the tumor 
followed by its regression and eventual disappearance. It will 
later be seen that their behavior in the present series of experi- 
ments is very similar to that in the earlier series referred to. 
Diagram SHowing RELATIONSHIP 
Common(N) x JAPANESE Warrz ING 
Py Hysrips 
Bac (ross 
(BC) 
Animals of series B.C., on the other hand, have one of their 
grandparents a Japanese waltzing mouse of the same inbred race 
which gave rise to the tumor, and one parent a first generation 
hybrid between the Japanese waltzing and common races. 
These first-generation hybrids, will, as has been shown by Tyzzer 
and the writer, grow the tumor as well if not better than ani- 
mals of the pure Japanese waltzing race. If, as seems certain, 
hereditary factors favoring growth of the tumor are introduced 
by the Japanese waltzing race, the B.C. generation has a direct 
opportunity to receive them, while the common race has not. 
