THE GENETICS OF TUMOR TRANSPLANTATION 291 



successful while transfer to unrelated strains is unsuccessful. This, of 

 course, also applies in the various transplanted tumors of mice which have 

 been studied. 



It is perhaps in the field of immunization that many of the most interest- 

 ing and important contributions by MacDowell and his associates have been 

 made. 



Although they recognize that much remains to be explained, and that 

 many complicating factors serve to obscure the true nature of the process, 

 they have made very definite progress to which some brief reference may be 

 made. 



By introducing very small numbers of leukemic cells — as, for example, 

 1/4,000,000 of the standard dose — the mouse may survive. If it does, it 

 shows that it can become modified to tolerate increasingly larger doses of 

 cells until it is finally able to "overcome massive doses of leukemic cells given 

 repeatedly." It is, however, clear that this immunity has no effect upon any 

 tendency to form spontaneous leukemia in the same animals. 



It is also interesting to note at this point that often the first transplants 

 of leukemic cells derived from a spontaneous growth, will not kill the host 

 before 20 to 90 days, while after a long series of transfers from mouse to 

 mouse, death may result in 3 or 4 days. This has also been the history of 

 some transplanted tumors, but not of all. 



In such "immunized" mice there is no trace of any antibodies in the 

 serum. This is in accord with the results obtained with transplanted tumors. 



Although there are no antibodies present "a susceptible host can be 

 immediately protected against a lethal dose of leukemic cells by treatment 

 with minced tissue from an actively immunized animal." Whatever causes 

 this protection is "intimately associated with living cells." By forcing the 

 minced tissue out of a syringe "held firmly against the bottom of the vessel" 

 all the cells are torn apart and the protective property is destroyed. Very 

 evidently these facts raise the possibility that some unknown mechanism of 

 resistance is involved. 



The injection of entirely normal tissue from an unrelated mouse may 

 also confer immunity. Various organs, both embryonic and adult, may 

 be used. Genetic constitution has a role to play. MacDowell states, how- 

 ever, that "before making the test there is no means of knowing the effect 

 of normal tissue of a given genetic constitution, except that the tissue of 

 the same genetic constitution as the host is ineft'ective." 



The resistance produced by a single treatment with normal tissue " difiers 

 from that induced by leukemic cells in that it cannot be passively transferred 



