96 LEONELL C. STRONG 



The decrease of susceptibility from youth to adult age has 

 been recognized by several investigators. In order to check 

 still further the increase of susceptibility with old age, one more 

 chart was drawn. The percentage reactions for class 5 with 

 the dBrB tumor were added to those with the dBrA tumor, 

 and likewise classes 2, 3, and 4 for the dBrA tumor reaction. 

 (This means merely the addition of the corresponding curves 

 on figures 6 and 10 — a curve of the first chart being added to its 

 analogous curve of the second chart.) Figure 11 shows the 

 results thus obtained. 



Class 5 112 negative : 30 reactions ±3.27 or 21.12% d= 2.30 



Classes 2, 3, 4 1387 negative : 80 reactions ±5.86 or 5.45% ± 0.40 



Difference 15.67% ± 2.35 



The difference is thus 6.67 times its probable error 



It will be noted that the two curves begin about the same 

 point — that is, that the initial percentage reactions for middle- 

 aged individuals (classes 2, 3, and 4, 8.17% ± 1.0) are about the 

 same as these for older mice (class 5, 13.5% ± 3.7). Individ- 

 uals in the full vigor of maturity are readily capable of coun- 

 teracting the tumor tissue. The number of positive reactions 

 gradually decrease to zero after the eighth observation (last 

 two points are not included in this chart, since only the mice 

 showing positive reactions were kept after the sixth observa- 

 tion). Old mice, however, apparently cannot counteract the 

 foreign tissue as readily as adult mice. The percentage reac- 

 tions gradually increase for the first few weeks, then gradually 

 decrease, until at the end of the sixth observation the old mice 

 show a greater number of positive reactions than they did at 

 the beginning of the experiment. As before stated, it is only 

 after several additional observations that the last palpable mass 

 from old mice disappeared. The increase in susceptibility is 

 real and not the result of a peculiarity of the tumor tissue 

 employed. It is evident that the old mice are approaching a 

 condition under which they are able to harbor the foreign tissue 

 better than they did when they were mature and reproductively 

 active. 



