SUSCEPTIBILITY TO TRANSPLANTABLE TUMORS 121 



2. That the growth rate of the transplanted tumor cell is 

 correlated with the number of genetic factors involved in the 

 reaction. This point has been previously discussed. This 

 hypothesis can be tested only by studying the growth rates for 

 back-crossed individualsi° where the factor of heterosis has been 

 partly eliminated. The last matter is still under investigation. 



The next point we desire to discuss is that outlined on pages 86 

 and 93 (fig. 8) . The two tumors here described have been under 

 observation for over a year. In that time it has been repeatedly 

 inoculated into wild house mice. During the whole course 

 of this experiment there has been no significant variation in the 

 reactive potentiality of either tumor (except Ex. N for dBrA). 

 The numbers employed are large enough to be significant and 

 the results cannot be disregarded on grounds of insufficient data. 

 We find no evidence for a rhythmic activity of the tumor cell. 

 In this connection we may recall the findings of Tyzzer and 

 Little. When the definitely proved homogeneous race of mice 

 are employed (Japanese waltzing mice), there has been no sig- 

 nificant fluctuation in the activity of the tumor cell in over 

 ten years. This tumor (carcinoma J. W. A.) remained true to 

 type throughout the whole length of the experiment. 



Several investigators, using laboratory stocks, claim to have 

 witnessed rhj^thmic fluctuations of the tumor cell, among whom 

 may be mentioned Haaland, Bashford, Murray, Bowen, and 

 Calkins. It may be suggested that the cyclic phenomenon wit- 

 nessed was the result of the second variable — that of the genetic 

 constitution of the varied races employed. 



S. Age 



One of the most interesting results that we obtained was 

 from the study of the susceptibility of individuals of different 

 age groups. The result is a confirmation of the previous con- 

 clusions of Little ('20). When a non-susceptible race is em- 

 ployed, susceptibility toward transplantable tissue decreases 

 with age up to the period of maturity. We have, however, 



10 Produced by crossing the Fi generation hybrids back to the susceptible 

 dilute brown stock. 



