HEREDITY AND TRANSPLANTATION OF TUMORS 369 



between American white and gray wild mice the differences between successive 

 generations noted in our first series did not occur. 



Subsequently Tyzzer and Little, and Little and Tyzer, found that a Japa- 

 nese mouse sarcoma, as well as a carcinoma, grew in F x hybrids between 

 Japanese and white mice as well as in pure Japanese mice, whereas in white 

 mice the tumors grew in only a small minority of the animals. The growth 

 of carcinoma and sarcoma behaved in these respects almost alike, but the 

 growth of sarcoma was somewhat better. Also, Tyzzer and Little interpreted 

 now, these variations in the percentage of takes in different generations of 

 hybrids as due to the action of multiple factors. They assumed that the con- 

 tinued growth of both the sarcoma and carcinoma depended upon the presence 

 of a complex of independently inherited factors, and this factor-complex was 

 supposed to be present in a nearly homozygous condition in the Japanese 

 waltzing mice. Since F x hybrids had Japanese mice as one of their parents, 

 they possessed the factors comprising the Japanese complex in a single dose, 

 and since this single dose allowed tumors to grow, it followed that a single 

 representation of these factors was all that was required for the establishment 

 of susceptibility to tumor implantation. Also, in his more recent investigations 

 Little assumed that the transplant must have double representation and the 

 host single representation of the genes required for continuous growth of the 

 grafted tumor. It was furthermore necessary to hold that inasmuch as there 

 was associated with the single complex of genes inducing susceptibility a set 

 of unlike genes in the host, the set of genes determining susceptibility was 

 dominant over the other set. Susceptibility was therefore supposed to be 

 dominant over non-susceptibility or resistance to the growth of transplanted 

 tumors. In addition, Tyzzer and Little assumed that the percentage of in- 

 dividuals in which the tumor grew in the F 2 generation could be used as an 

 index of the number of factors necessary for continued growth. The larger 

 the number of F 2 hybrids in which the tumor takes, the fewer the number of 

 genetic factors required. For instance, these authors concluded that twelve 

 factors were necessary in the case of a carcinoma, and from five to seven in 

 the case of a more readily growing sarcoma. 



In subsequent investigations into the transplantability of tumors, Little and 

 Strong made use of strains which had been rendered more or less homogene- 

 ous (homozygous) by means of long-continued sister-brother inbreeding. In 

 transplanting a melanotic tumor, which originated in strain dba, into hybrids 

 between dba and A, Spangler, Murray and Little noted that transplantations 

 succeeded in a larger percentage in colored than in albino hosts, and they 

 assumed that one "susceptibility factor" is required for a successful trans- 

 plantation in colored mice, while in albino mice there is needed, in addition, a 

 second factor, which would be necessary also for melanin production in non- 

 colored individuals but would function in this way only in the presence of the 

 color factor. The use of closely inbred strains meant, in certain respects, a 

 great simplification of the analysis of the growth of transplanted tumors and 

 led to the establishment of some important facts in a more definite manner 

 than had been previously possible. To mention only one example : Bittner, by 



