236 C. p. RHOADS 



Recent studies by Tatum {'jy) bear on this point. A mutant strain 

 of Neurospora unable to synthesize nicotinic acid accumulates a special 

 pigment, never formed by its progenitor, the wild type organism which 

 can synthesize from nitrogen and sugar all of its components except 

 biotin. The pigment is formed from an unused precursor of nicotinic 

 acid. It is not the precursor itself, but rather a product of conversion of 

 the precursor to pigment by an enzyme not demonstrable in the wild type 

 mold. By a mutant blocking of one system, that for synthesizing nico- 

 tinic acid from the precursor, a new enzyme previously unused because 

 unneeded, comes into play. The apparent appearance of a new function 

 is largely the disappearance of an old one. 



Concerning the abnormal functions of cells, particularly those of 

 neoplastic tissue, a generally similar but abnormal process conceivably 

 could have taken place with the production of variants possessing wholly 

 unique abilities. For such a cell to survive requires that it not only be 

 successful in competition with the normal cells in the company of which 

 it finds itself, but also not susceptible to injury by whatever general 

 mechanisms are possessed by the body for the control of cell growth. 



Genetics and the Cancer Cell 



The material presented provides a basis for a consideration of cellu- 

 lar differentiation, normal and abnormal, in terms of the physiology of 

 simple forms. If differences between cells are genetic, they must be 

 due to somatic mutation, the alteration of genes. No final proof of gene 

 effects in the production of cancer is possible, since tissue cells are 

 vegetative and not sexual, and since chromosome changes are not regu- 

 larly demonstrable. Mutation is believed to be a possible cause because 

 cancer can and does arise in localized areas, perhaps from single cells, 

 and it results regularly from agents known to cause mutation. Since 

 genes control specific steps in metabolism, they must have altered in 

 the production of neoplastic cells of abnormal composition. Beyond 

 this point of speculation we can do little more than argue from analogy. 

 It may be useful, however, to set down what is known about how cancer 

 cells differ from their normal prototypes. 



The earliest experimental information concerning the cancer cell 

 came from the demonstration that under certain circumstances it could 

 be transplanted into otherwise normal hosts. This naturally required 

 reference to the rules which apply to the transplantation of normal 

 tissue from one animal to another. In general, an animal will not accept 



