SUSCEPTIBILITY TO TRANSPLANTABLE TUMORS 91 



positive reactions, usuall}^ died, possibly because the experi- 

 ment was performed during the epidemic previously mentioned. 

 The young mice either died of the infection or because they 

 were deprived of their foster-mother, or because the reaction 

 (host-tumor) was too virulent for the young mice. But even 

 taking this complicating factor into account, there is still a 

 significant increase in the percentage reactions of the very 

 young (class 1) individuals (fig. 6, p. 89). 



Class 1 134 negative : 30 reactions ±3.34 or 18.29% ± 2.03 



Class 2, 3 and 4 714 negative : 54 reactions 4.73 or 7.03% ± 0.61 



Difference 11.26% ± 2.12 



The difference is thus 5.32 times its probable error 



The numbers for the very young wild mice are small, but 

 even in spite of this, the difference between class 1 and 2, 3, and 

 4 is significant. The fact that very young individuals of anon- 

 susceptible race are more susceptible to transplantable tissue 

 has been recognized by several investigators, among whom may 

 be mentioned Tyzzer, Little, and Loeb. We may next consider 

 the reaction of old mice to the tumor. 



As far as known to us, the following experiment is the first 

 reported observation on susceptibility to transplantable tissue 

 in very old mice of a non-susceptible race (class 5). Contrary 

 to what we expected, susceptibility increased from maturity 

 toward old age. 



Figure 6 shows the three middle-class curve (heavy solid 

 line) compared with that for the old (class 5) mice (dotted line) 

 as well as that for very young (class 1) individuals (light solid 

 line). 



Tabulated data 

 Class 5 57 negative : 18 reactions ±2.49 or 24.00% ± 3.30 



Class 2, 3, and 4 714 negative : 54 reactions ±4.73 or 7.03% ± 0.61 



Difference 16.97% ± 3.35 



The difference is thus 5.07 times its probable error 



Two points of especial interest may be emphasized at this 

 time: 1) The susceptibility toward transplantable neoplastic 

 tissue is as great in very old mice as it is in very young mice 



