SUSCEPTIBILITY TO TRANSPLANTABLE TUMORS 129 



of visible reactions. The mouse slowly recovers its normal 

 physiological activity, and with this gradual recovery it is more 

 capable of producing an increasing percentage of reactions. By 

 keeping individuals for 120 days or longer between operations 

 and inoculation, the process of senescence becomes mechanically 

 an important factor (some of these individuals were gonadec- 

 tomized when adult). The decreased metabolic activity of old 

 age is probably an explanation of the decrease in the percentage 

 reactions, as stated above (it will be recalled, however, that for 

 normal individuals susceptibility to transplantable tumors 

 increased with old age — just the opposite from that obtained with 

 gonadectomized old mice. This point will be considered later). 



Previous investigators have not taken into consideration the 

 deleterious effect of the operation. It has already been pointed 

 out that results obtained with non-susceptible mice are the exact 

 opposite of those obtained with susceptible strains. Corson- 

 White and other investigators have encountered an increased 

 percentage indication by castration (working with a partially 

 susceptible race). Could not this result also be explained by 

 maintaining that the operation alone had a stimulating effect on 

 susceptibility? 



Our next experiment dealt with the actual effect of gonadec- 

 tomy itself. After correcting for the harmful effect of the opera- 

 tion alone, we have been unable to observe any difference in the 

 number of percentage reactions between the controls and gonad- 

 ectomized individuals (fig. 21, p. 105, and others). Gonadectomy 

 neither increases nor decreases the number of reactions for the 

 six-week observation period, when all age groups are massed. 



This result is to be expected. The number of susceptible 

 individuals in any given race of mice is determined by genetic 

 factors primarily. There are, however, several secondary fac- 

 tors (such as age, etc.) that determine whether the reaction to the 

 tumor is to be prolonged or shortened. 



The one noticeable effect of the removal of the gonads is the 

 fact that a certain small number of animals continue to grow the 

 tumor mass progressively. We cannot conclude from this result 

 that gonadectomy increases the rate of tumor growth. The 



