224 C. P. RHOADS 



spora have proven beyond any question that the enzymatic activities of 

 those cells and, by inference, cells of any type, are formed by genes. 

 Beadle states as follows : "Genes evidently determine the final potentiali- 

 ties of differentiation in a given organism. Whereas the initiation of the 

 orderly process may be through the environment, whether or not a given 

 reaction . . . can take place is gene controlled." An example is at hand in 

 the MacDowell (29) dwarf mice, in which the normal gene concerned is 

 responsible for the development of pituitary eosinophils. Without this 

 gene the pituitaries of the dwarf animals lack the eosinophilic com- 

 ponent which, if supplied artificially, restores growth. Because of this 

 lack the growth extent and rate of all organs is enormously reduced. 



If the control of normal cellular development and differentiation by 

 genes is indicated by the evidence for genie regulation of protein and 

 enzyme production as well as for the existence of spontaneous varia- 

 tions in genes, and if a similar mechanism is to be invoked in the con- 

 trol of abnormal differentiation, as of the cancer cell, it becomes neces- 

 sary to establish the possibility of ahnornml mutations. Mammalian cells 

 are vegetative and hence not susceptible to genetic study by standard 

 methods. For this reason, the last step possible in a correlation of ab- 

 normal variation with cancer is to show that the agents which produce 

 cancer cells also cause mutations in sexual forms. Mutations can be 

 produced artificially by certain physical and chemical agents, and appar- 

 ently by proteins with the properties of antisera. The physical agents 

 include x-rays, gamma rays, neutrons, and ultraviolet light, and a mass 

 of evidence proves that all of these are carcinogenic. 



The Artificial Production of Gene Changes 

 AND Cancer by Physical Agents 



The studies of mutations as caused by x-rays are classic. Chromosome 

 aberrations are said to result from x-rays more frequently than they 

 occur spontaneously (30), but true mutations of the spontaneous types 

 are also caused. Many have been proven to be due to the removal of 

 small segments of chromosomes followed by junction of the broken 

 ends. Some cannot be due to such a chromosome deficiency, however, 

 and rather must represent an actual change in the composition of the 

 gene, since back mutations occur. The mutations appear to start as 

 changes in individual atoms, in consequence of which, by a chain of re- 

 actions, the mutation results. The question of whether gene mutation 



