120 LEONELL C. STRONG 



therefore, that this exceptional case was produced by the 'adapta- 

 bihty' of the tumor cell is not very great. The W238 dBrA 

 tumor should have possessed an increased power of adaptation, 

 since it had already grown in one foreign soil for several weeks. 

 The inference has already been drawn that perhaps the reaction 

 potentiality of the tumor cell is an expression of its genetic 

 make-up. If this is the case, we can explain the remarkable 

 increase of growth capacity in the W238 dBrA tumor by main- 

 taining that there must have been some marked change of the 

 tumor cell. We wish to emphasize the fact that this change 

 could have been produced by some process analogous to that 

 giving rise to 'mutations,' that occur in all normal types of animal 

 and plant tissues. 



In our own experiment we have been able to verify the previous 

 findings of Tyzzer relative to the growth rate, and in addition 

 can suggest an explanation. The Fi hybrids obtained between 

 dilute brown (susceptible) and albino (non-susceptible) mice 

 grew the two transplantable tumors, dBrA, dBrB, progressively. 

 The growth curve for the Fi's produce about three to four times 

 the mass of tissue that the controls do in the same length of 

 time. Two interpretations suggest themselves: 1. That hybrid 

 individuals grow the tumor tissue faster because of the phenom- 

 enon of heterosis (hybrid vigor) . Heterosis is a common occur- 

 rence encountered in the crossing of many diverse strains 

 of individuals within a species. The exact mechanism that 

 brings about this increased vigor is still in dispute (East and 

 Jones, Inbreeding and Outbreeding). The observed fact is, 

 that by crossing two diverse strains, individuals are frequently 

 produced that are more active and vigorous than either parent 

 strain that entered into the cross. Thus, Fi hybrids are found 

 to be more hardy, healthy, and more resistant to diseases. It 

 is, however, not evident why the increased growth rate on the 

 part of the tumor cell should be correlated with the increased 

 vigor of the Fi host unless the general physiological condition 

 of the animal affects also the fate of tumor transplants on it.' 



' Health and other factors bearing upon the physiological condition of the 

 host do affect the transplantation of tissues. 



