406 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



merely to quantitative differences in the intensity of the immune reactions in 

 various animals or strains, in the ability of different kinds of tumors to neu- 

 tralize the immune substances, and in the power of resistance of different 

 tumors to the injurious action of such substances. The immunity found after 

 extirpation of a growing tumor or after retrogression of a formerly growing 

 tumor would then represent merely quantitative variants of the same type of 

 immunity. 



There exist certain other experimental procedures which may make it possi- 

 ble to prove the existence of immunity against a tumor graft. This may in 

 some cases be accomplished by experimentally weakening the second tumor in 

 vitro, previous to inoculation. Also, by artificially weakening a first tumor it 

 may be possible to demonstrate the development of an active immunity, be- 

 cause under these conditions the absorbing and neutralizing function of the 

 tumor may be markedly diminished. Through experimental weakening of the 

 second tumor by means of graded application of heat previous to transplan- 

 tation, Fleisher, Corson-White and the writer demonstrated the inhibiting 

 effect of the first tumor on the development of a subsequently transplanted 

 tumor, in mouse carcinoma No. IX, in which, in successive transplantations 

 of fully active, unheated tumor pieces, immune processes are not manifest. 

 Thus it could be shown that a first unheated tumor possessing its full growth 

 energy prevents the growth of a second tumor which has been exposed to a 

 temperature of 44° for a period of from thirty-five to forty minutes. It does 

 not entirely suppress, but it weakens the growth of a second tumor which has 

 been exposed to a temperature of 44° for thirty minutes. If the growth energy 

 of the first tumor has also been slightly reduced through heating, the develop- 

 ment of a second tumor is prevented only if its growth energy has been dimin- 

 ished quite markedly through heating for forty minutes. But if the first tumor 

 had been injured through heating as much as the second tumor, or even more, 

 we then observed in several instances the opposite phenomenon, namely, an 

 increase in the growth energy of the second tumor. Thus one tumor may, under 

 certain conditions, have a beneficial influence on the growth of a second tumor, 

 perhaps owing to a neutralizing effect on substances antagonistic to tumor 

 growth which a first, weakly-growing tumor may exert. 



It seems that the antigenic function of a tumor graft bears some relation to 

 the intensity of its metabolic activity, or to the presence of substances which 

 are readily injured by heat even of a moderate intensity. The influence which 

 the second tumor exerts on the first is less marked, but an enhancing effect of 

 a second, less inhibited tumor on a first, weakened tumor has been observed 

 also by Andervont in the case of sarcoma 180. Under other circumstances, 

 however, a second tumor whose growth energy has been only moderately 

 diminished through heating may be victorious in competition with a first, more 

 markedly depressed tumor ; and it is further possible to produce experimental- 

 ly a balancing between a first and a second tumor. Apparently the interaction 

 of two mutually antagonistic processes may play a role in bringing about this 

 effect, namely, (1) the production of immune substances in the host, and 

 (2) their absorption and neutralization by the tumor, or perhaps by organismal 



