254 C. p. RHOADS 



work in botany and in embryology. The substances described as neo- 

 plastic viruses have not been shown to possess any single characteristic 

 which is not shared by substances which are not infectious parasitic 

 agents. Particle size, antigenicity, protein or even nucleo-protein com- 

 position, autocatalytic propagation, inactivation by heat, and changes 

 of pH are ubiquitous. Transmission by the maternal cytoplasm or by 

 nursing are all shared by other, non-viral, substances. With all the vast 

 amount of experimentation and highly illuminating, indeed brilliant, 

 results, perhaps transcending in importance those of any other field of 

 biology, the expression coined by Murphy for the Rous agent of "trans- 

 missible mutagen" seems to cover the facts more adequately than any 

 other. 



Summary 



A discussion has been presented of certain mechanisms known to 

 cause self-perpetuating alterations of cells in general. This has been 

 done because of the evidence that neoplastic abnormal growth is due to 

 unique, inherited properties of its constituent cells. It is hoped that from 

 a scrutiny of a variety of data, in part apparently irrelevant to the can- 

 cer problem, certain lines of investigation will stand out as potentially 

 profitable in the future. 



Evidence has been advanced that the neoplastic process is a distinc- 

 tive, characteristic sort of abnormal growth, malignant as well as autono- 

 mous, something more than a quantitative deviation from the normal 

 rate and extent of differentiation. 



Reference has been made to the factors which control normal growth 

 and changes in cells to ascertain to what extent principles normally 

 operative can be invoked to explain the acquisition of abnormal prop- 

 erties. It is clear that the constituents of cells which vary either normally 

 or pathologically are under the control of genes to a very great extent. 

 Gene mutation, either spontaneous or induced, is held to be able to effect 

 the types of changes of cell composition and function, normal and ab- 

 normal, which have been described. This principle of genie control is 

 invoked as applicable to self-perpetuating cytoplasmic factors and to 

 maternal influences, as well as to cell constituents known to be the direct 

 product of the gene. The evidence that the ability of substances formed 

 by the organs of internal secretion to affect the growth and differentia- 

 tion of cells depends upon the inherited properties of the target tissue 

 has been discussed. 



Attention has been called to the correlation between the ability of cer- 



