SUSCEPTIBILITY TO TRANSPLANTABLE TUMORS 131 



times the mouse is inoculated, nor how long the mouse is kept 

 between operation and inoculation. If very old animals are 

 castrated, the same result is obtained. Operated old individuals 

 give no visible reaction masses whatever. The maximum effect 

 of the removal of the gonads is produced on those individuals 

 that are at the moment of acquiring sexual maturity. It may 

 be recalled that age group 3 for the controls gave the minimum 

 number of reaction masses. The age-susceptibility curve for 

 operated individuals is not the same as the a&o-susceptibility 

 curve for the controls of a non-susceptible race. Gonadectomized 

 individuals (from a non-susceptible race) give a susceptibility 

 curve resembling that characteristic of individuals from a sus- 

 ceptible strain. The last end of the age-susceptibility curve 

 for a susceptible race has not been definitely determined, but it 

 seems likely that it will be found that in such a race susceptibility 

 to a transplantable tumor decreases with old age. The greatest 

 effect of gonadectomy is produced on individuals that are not 

 only undergoing sexual maturity, but also the correlative proc- 

 ess of physiological differentiation. 



The failure of old gonadectomized individuals to show any 

 reactions to implants may be explained, partly on the decreased 

 physiological activity cf old animals, and partly as a result of 

 the genetic con.position which they in common with all adult 

 animals of their race possess. Certain physiological agents cease 

 to function with the onset of senescence. The shock of the 

 operation is severe enough to reduce considerably the normal 

 phj^siological activity of the organism. Before the shock of the 

 operation is over, so that the normal physiological activity of 

 old mice can be restored, the final lowering of this activity due to 

 old age has set in. The animal is therefore merely able to keep 

 alive for a short time and cannot nourish the implanted tissue. 

 The host has therefore become again a relatively passive element, 

 and the tumor, no longer calling forth any reaction, neces- 

 sarily dies. 



Another point of minor importance is the fact that the effect of 

 gonadectomy upon the two sexes is not different, thus indicating 

 that the gonads are a common factor in the general processes 

 of assumption of tissue specificity for both sexes. 



