226 C. P. RHOADS 



the mutation has occurred in hypothetical nuclear genes or in hypothet- 

 ical cytoplasmic determiners or plasmagenes. 



It is entertaining to speculate on cellular differentiation as a progres- 

 sive series of losses of function, like those undergone by mutated Neuro- 

 spora, by which the cell retains the ability to exercise a certain number 

 of special functions while, through mutation, the abilities are lost to 

 carry out certain other activities to completion. The formation of pig- 

 ment, for example, or the accumulation of any metabolite, could be 

 simply the result of loss of ability to carry out completely a series of 

 metabolic steps. 



The simple gene-hit theory of x-ray effects has been brought into 

 question by the experiments reported by Dale (35) on the changes in- 

 duced by x-rays of two enzymes in solution. These suggest that the 

 molecules of the solvent are activated by the absorption of radiation 

 and transfer their energy to the dissolved protein. The fact that enzymes 

 can be so inactivated suggests the existence of an intracellular effect of 

 radiation on enzymes somewhat more complex than is explainable by the 

 simple gene-hit theory. The work on Neurospora proves that a direct 

 relationship exists between genes and enzymes. Hence the Dale mech- 

 anism may simply prove the existence of an indirect route, for the x-ray 

 effect, through enzymes to genes. Lea, Smith, Holmes, and Markham 

 (36) have provided further evidence on this point by showing that virus 

 inactivation by radiation may be either direct, independent of concentra- 

 tion of the solution, or indirect in dilute solution. 



The ability of x-rays to cause cancer has been amply established by 

 tragic experience among radiologists and requires little documentation. 

 Particularly detailed confirmation is found in the recent experimental 

 studies of Henshaw (37) on the production of leukemia in mice by 

 irradiation. Furth (38) has reported that granulosa cell tumors of the 

 ovary can be similarly caused in mice. A most interesting observation 

 on the neoplastic effects of x-rays on man is that of March (39), who 

 finds that the incidence of leukemia in radiologists is more than ten times 

 that among physicians who are not radiologists. 



Ultraviolet light, also a cause of cancer, seems to be somewhat less 

 effective than are x-rays in the induction of mutation. The effects of the 

 two agents on maize were carefully compared by Stadler (40), who 

 found certain differences. An explanation may be at hand in the length 

 of the time interval between the break and the separation of the chromo- 

 some fragments when the mutation is due to ultraviolet. The striking 

 effect of certain wave lengths of the ultraviolet is in sharp contrast to 



