MANNER OF PRODUCTION OF MUTATIONS 585 



elusion was reenforced by the findings of King that variations in the 

 relative amount of stable P^^ accompany'ng the radioactive P^- did not 

 alter the mutagenicity when the concentration of administered P^^ was 

 kept constant, despite the change w^hich must thereby have been brought 

 about in the amount of radioactive P^'^ absorbed by the organisms. 

 (Here the paths of the 13 rays are so long that the great majority of the 

 ionizing particles within the flies penetrate them from outside.) This 

 negative result, nevertheless, fails to prove that changes in the nuclei of 

 phosphorus atoms within the chromosome are 7iot mutagenic. One 

 reason is that in this work the substance was fed to adult flies in which 

 the chromosomes of the spermatozoa, from which the tested offspring 

 were derived, may already have been formed prior to treatment. 



The work of Hungate and Mannell (1951, 1952) on the mutagenic effect 

 of radioactive sulfur, S*^, on growing Neurospora further illustrates the 

 difficulties in interpretation of the results of such experiments. These 

 investigators found that, when a given constant amount of S^^ is present 

 in the medium, the mutation rate is inversely correlated with the amount 

 of stable sulfur, S^-, which has simultaneously been administered. They 

 have regarded this as indicating that the additional mutations, occurring 

 in the presence of less S^^, were caused by the transmutation of S^^ into 

 CP^ within the genes. The calculations were not simple, however, since 

 there was a very large and not directly determined amount of "contami- 

 nating" stable sulfur present in the medium, of unknown utilizability. 

 Nevertheless, determinations of the radioactive sulfur within the Neuro- 

 spora showed that in most of the cultures, despite the different amounts 

 of intentionally added S''^^, the amount of S^^ within the cells remained 

 about the same. Since the S'^^ outside the cells must have been constant 

 also, it was concluded that the amount of ionizing radiation in the cells 

 was likewise equal in these cases. Thus the observed differences in the 

 mutation rates were judged not to be due to differences in the ionizing 

 radiation to which the cells were subjected but to differences in the fre- 

 quency of transmutation within their genes. 



At the same time, the investigators believe that the mutations arising 

 included a much higher frequency than usual of biochemical deficiencies 

 of a peculiar sort, in which one amino acid will do duty for another. 

 This result, if substantiated by comparison with exact controls, would 

 bespeak the distinctiveness of the mutagenic process here involved, as 

 compared with that induced by ionizing radiation. 



Before acceptance of the transmutation interpretation, however, several 

 other possibilities have had to be disposed of. All these are connected 

 with the inference that, as Hungate and Mannell point out, the cells 

 in the media to which more S^- had been added probably contained 

 more total sulfur. Since they had about the same concentration of S^^, 

 despite the added S^-, and would not be expected to exercise any con- 



