134 CARNEGIE INSTITUTION OF WASHINGTON. 



"The sex chromosome has been ehminated as a probable carrier of any of 

 the four factors, as follows : If mice, like other mammals, have the female XX 

 and the male XY in formula, the use of susceptible Japanese waltzing males 

 to form the Fj animals used gives daughters carrying his X and sons his Y 

 chromosomes. If, now, his sojis only are used to produce the back-cross 

 generation by mating with common non-susceptible females, all the chromo- 

 somes in the resulting animals will be derived from common non-susceptible 

 mice and the male offspring would be all non-susceptible; but such is not the 

 case. Unless, therefore, crossing-over between the X and Y chromosomes 

 occurs frequently, any susceptibility factor borne in the X chromosome of the 

 original Japanese waltzing males used has been eliminated. 



"While further investigations are in progress, we may conclude provisionally 

 that: 



" (1) From three to five factors, probably four, are involved in determining 

 susceptibiUty to the mouse sarcoma J. w. B.; (2) that for susceptibihty the 

 simultaneous presence of these factors is necessary; (3) that none of these 

 factors is carried in the sex (X) chromosome ; and (4) that these factors men- 

 delize independently of one another. 



"Factors underlying growth of a transplantable mouse sarcoma (J. w. B.) — 

 A series of experiments was undertaken by Little to determine factors that 

 underhe sarcoma growth. The 675 mice used in these experiments were of 

 two races: (1) common non-waltzing animals of albino (a) and dilute brown 

 (dbr) stocks, and (2) hybrids produced by crossing these common races with 

 Japanese waltzing mice and then back-crossing the first-generation hybrid 

 with the common non-waltzing parent race. 



"The two races are very different biologically. Series N includes the 

 common-stock mice unrelated 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 Little have shown there may be temporary growth of 

 the tumor followed by its regression and eventual disappearance. Their 

 behavior in the present series of experiments is very similar to that in the 

 earlier series referred to. 



"Animals of series B. C. (back-cross), 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 shown by 

 Tyzzer and Little, grow the tumor as well as, if not better than, animals of the pure 

 Japanese waltzing race. If, as seems certain, hereditary factors favoring growi;h 

 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. 



"Beginning with a date two weeks after inoculation, weekly observations 

 were made upon all inoculated mice. The mice were examined individually 

 by palpation, and the presence or absence of a mass noted. If a mass is present 

 it is described, and if it is larger than a pinhead a sketch is made of it on the 

 record sheet of the mouse. 



"Thus weekly observations are made in the case of all animals up to and 

 including the sixth week after inoculation. From that time on, observations 

 are made upon only those animals showing a mass. In this way a diagram- 

 matic representation is obtained of the gradual growth of the tumor as well as 

 a record of its diminution and eventual disappearance, should this take place. 

 The absolute size of the mass can not, of course, be very accurately deter- 

 mined, and is not to be considered at present. The ]\Iisses B. W. Johnson, 

 E. E. Jones, and D. M. Newman have been of the greatest assistance in the 

 tedious work of inoculation and observation of the mice. 



