92 LEONELL C. STRONG 



(24.00% ± 3.30 compared with 18.29% d= 2.03) and 2). The 

 siisceptibiUty curve for old mice does not approach zero during 

 the usual time limit of seven weeks as do all the other suscepti- 

 bility curves so far given. Old mice are extremely slow in elimi- 

 nating the last traces of the tumor reaction mass. As a matter 

 of fact, some old mice took from eleven to twelve weeks to over- 

 come finally this reaction (host-tumor) . It may be recalled that 

 some reaction masses become encysted and remain apparently 

 quiescent for a considerable period. The time element for the 

 actual reaction can only be determined in those cases in which 

 there is some change in the relative size of the mass from week 

 to week. 



e. Age Siisceptihilty {clBrB). We are now able to plot the relative 

 percentage reactions for the five age groups. The curve shows at 

 a glance the susceptibility of animals of a non-susceptible race 

 at different periods in their life (fig. 7, light solid line, dBrB). 



When individuals (of the non-susceptible race) are very young 

 they are highly susceptible to the transplanted tissue — about 

 one-fifth of all the observations for the six weeks of the experi- 

 ment are positive visible reactions. As the individual grows 

 older there is at first a rapid decline in susceptibility until the 

 second class is reached, then a slower decline until the mouse 

 is about three months old. The lessened susceptibility then 

 remains fairly uniform throughout the adult stage. With senes- 

 cence and old age, the susceptibility rapidly increases until the 

 original maximum of about 25 per cent is reached. The tw^o 

 points in this curve call for emphasis. 1) Individuals that are 

 one-half to three-fourths matured are less susceptible to trans- 

 plantable tissue than are individuals from any other age groups. 

 This is the exact converse of w^hat has been recognized by some 

 investigators as applying to a susceptible race. 2) The age 

 susceptibility toward a transplantable tissue bears a remarkable 

 parallelism to the activity of the gonads. 



Before the gonads mature, susceptibility toward tumor tissue 

 is very high (although we are dealing with a non-susceptible 

 race). During the period of the development of the sex organs 

 (classes 2 and 3), susceptibility at first rapidly, and then grad- 



