SUSCEPTIBILITi' TO TRANSPLANTABLE TUMORS 125 



sexual maturity is definitely reached, the sexes remain fairly 

 uniform in their reactive capacities because maturity is a period 

 of relatively slight physiological change then, in both sexes. 

 We should therefore expect, under this assumption, that adult 

 individuals would show the same reactive potentiality regard- 

 less of sex (which is the observed result) . 



By the use of a preponderant number of adult individuals one 

 would, in any experiment, be unable to detect the influence of a 

 sex factor; the result would be that large numbers of animals of 

 all ages would mask an actual sex difference for individuals of 

 the special age groups where there is a significant difference be- 

 tween the sexes. 



What is to be expected when senescence sets in? Do the 

 gonads begin to decrease in physiological activity at the same 

 period of life in both sexes? The period of maturity for male 

 mice is relatively shorter than the corresponding period for 

 females. It has already been pointed out that in a non-sus- 

 ceptible race susceptibility increases with old age. This increase 

 in our experiments is almost exclusively confined to the male 

 sex, so that if old mice alone are inoculated there would be a 

 significant difference in susceptibility between the sexes. The 

 sex factor can therefore be largely explained as the result of the 

 primary factor of age. 



The phenomena encountered in studying susceptibility in a 

 non-susceptible race are mostly the exact opposite of those found 

 to hold true for susceptible races. For example, susceptibility 

 to transplantable tissue decreases with age up to maturity for 

 non-susceptible individuals, whereas it increases for susceptible 

 animals for the same age group. We may therefore expect 

 that in both cases the sex-factor difference for very young and 

 very old individuals will be partly explained by the degree of 

 physiological specificity of the tissue which the individual pos- 

 sesses at the time of inoculation. Female mice, maturing faster 

 than males, are able more quickly to counteract the activity of 

 the tumor. 



From a genetic view-point, the tumor cell as well as the host 

 element must be taken into consideration. It is reasonable to 



