256 C. p. RHOADS 



bodies can be induced in distantly related hosts by the inoculation of 

 tumor material. From the evidence, however, cancer of spontaneous 

 origin is not productive of antibodies capable of controlling the growth. 

 Some other factor appears to play a role in immunity, and a particularly 

 mysterious and important one. Intensive study is indicated of the anti- 

 genic constituents of the neoplasms induced by the conventional physical 

 and chemical agents as well as of those neoplasms known to follow the 

 inoculation of special proteins. 



There is believed to be no fundamental incompatibility between the 

 thesis that some cancers are due to self -perpetuating, perhaps cytoplas- 

 mic, protein components, and the contention that most are the result of 

 gene change and represent somatic mutations. The dividing line between 

 these two mechanisms has become so indistinct, so much interrelation- 

 ship between nuclear genes and cytoplasmic constituents has been proved 

 to exist, and such a vast number of different kinds of self -perpetuating 

 entities have been described, that an agent can be found to suit almost 

 any taste. 



Some cancers can be transmitted by filtrates and continue to produce 

 the active material, some result from filtrates but yield none of the eti- 

 ologic substance, some may conceivably result from altered enzymes, 

 either directly, or indirectly through their interaction with genes, and 

 some from direct gene hits with the production of standard mutations. 

 In spite of all these possibilities and the immense number of literary 

 and scientific contributions to the subject, there appears no conclusive 

 proof that any mammalian cancer is caused by an intracellular parasite 

 completely foreign to the host. 



Reference has been made to the generalized effects upon cellular dif- 

 ferentiation, including the production of cancer, which follow the sys- 

 temic administration of certain compounds with hormone activity and 

 also of special chemical substances not known to have hormone effects. 

 The possibility has been advanced that perhaps one step in the produc- 

 tion of neoplastic abnormal growth may be the chronic exposure of cells 

 to an intolerable environment, with consequent death or defensive vari- 

 ation and the development of protective mechanisms. Such an environ- 

 ment might well allow the variants which occur spontaneously, if they 

 are equipped with protection, to persist and overgrow the normal cells. 

 Since the standard controls of growth are established to govern only 

 standard cells, certain variant cells might not be susceptible to them, 

 and, if so, might grow without restraint. This concept of neoplastic 

 growth will of course be immediately opposed by those who envision 



