MULLER 



From the standpoint of biological 

 theor\% the chief interest of the present 

 experiments lies in their bearing on 

 the problems of the composition and 

 behavior of chromosomes and genes. 

 Through special genetic methods it 

 has been possible to obtain some in- 

 formation concerning the manner of 

 distribution of the transmuted genes 

 amongst the cells of the first and later 

 zygote generations following treat- 

 ment. It is found that the mutation 

 does not usually involve a permanent 

 alteration of all of the gene substance 

 present at a given chromosome locus 

 at the time of treatment, but either 

 afi^ects in this wav^ only a portion of 

 that substance, or else occurs subse- 

 quently, as an after-effect, in only one 

 of two or more descendant genes de- 

 rived from the treated gene. An ex- 

 tensive series of experiments, now in 

 project, will be necessary for deciding 

 conclusively between these two pos- 

 sibilities, but such evidence as is al- 

 ready at hand speaks rather in favor 

 of the former. This would imply a 

 somewhat compound structure for the 

 gene (or chromosome as a whole) in 

 the sperm cell. On the other hand, the 

 mutated tissue is distributed in a man- 

 ner that seems inconsistent with a gen- 

 eral applicability of the theory of 

 "gene elements" first suggested by An- 

 derson in connection with variegated 

 pericarp in maize, then taken up by 

 Eyster, and recently reenforced by 

 Demerec in Drosophila virilis. 



A precociously doubled (or further 

 multiplied) condition of the chromo- 

 somes (in "preparation" for later 

 mitoses) is all that is necessary to ac- 

 count for the above-mentioned frac- 

 tional effect of X-rays on a given 

 locus; but the theory of a divided 

 condition of each gene, into a number 

 of (originally identical) "elements" 

 that can become separated somewhat 

 indeterminately at mitosis, would lead 



153 



to expectations different from the re- 

 sults that have been obtained in the 

 present work. It should, on that 

 theory, often have been found here, as 

 in the variegated corn and the ever- 

 sporting races of D. virilis, that mu- 

 tated tissue gives rise to normal by 

 frequent "reverse mutation"; more- 

 over, treated tissues not at first show- 

 ing a mutation might frequently give 

 rise to one, through a "sorting out" of 

 diverse elements, several generations 

 after treatment. Neither of these 

 effects was found. As has been men- 

 tioned, the mutants were found to be 

 stable through several generations, in 

 the great majority of cases at least. 

 Hundreds of non-mutated descendants 

 of treated germ cells, also, were carried 

 through several generations, without 

 evidence appearing of the production 

 of mutations in generations subsequent 

 to the first. Larger numbers will be 

 desirable here, however, and further 

 experiments of a different type have 

 also been planned in the attack on this 

 problem of gene structure, which 

 probably can be answered definitely. 

 Certain of the above points which 

 have already been determined, espe- 

 cially that of the fractional effect of 

 X-rays, taken in conjunction with that 

 of the production of dominant lethals, 

 seem to give a clue to the especially 

 destructive action of X-rays on tissues 

 in which, as in cancer, embryonic and 

 epidermal tissues, the cells undergo 

 repeated divisions (though the opera- 

 tion of additional factors, e.g., abnor- 

 mal mitoses, tending towards the same 

 result, is not thereby precluded); 

 moreover, the converse effect of 

 X-rays, in occasionally producing 

 cancer, may also be associated with 

 their action in producing mutations. 

 It would be premature, however, at 

 this time to consider in detail the vari- 

 ous X-ray effects previously con- 

 sidered as "physiological," which may 



