MANNER OF PRODUCTION OF MUTATIONS 483 



restitute, are on far more doubtful ground; for they are based on a 

 number of assumptions which are as yet unverifiable and which seem 

 dubious from a biological point of view. 



In further investigation of the production of gross structural changes 

 involving two or more breaks, the effect of variations in the timing of the 

 irradiation applied to mature Drosophila spermatozoa was studied. It 

 was found that radical differences in timing, brought about in one case by 

 dividing the dose into four fractions separated by intervals of a week and 

 in another case by extending it throughout a month at an intensity of 

 0.01 r/minute, as compared with a single concentrated treatment given 

 at 100 r/minute, caused no difference in the frequency of the transloca- 

 tions produced (see Muller, 1938, 1939a, c, 1940; Makhijani, 1945). It 

 was therefore concluded that, in the case of all the breaks produced by 

 independent hits, the broken ends must remain open (ununited) as long 

 as the chromosomes are in the condensed condition; for, if they had been 

 able to undergo union before fertilization occurred, then when the long 

 protracted or widely divided treatments were given, some of the broken 

 ends would have undergone restitution before having a chance to unite 

 with the ends derived from breaks which were produced later, and these 

 treatments would therefore have yielded fewer structural changes than 

 the concentrated ones. These findings were later confirmed by others 

 (e.g., Dempster, 1941a; Kaufmann, 1941), but experiments of Catsch 

 and Radu (1943) indicated that, in spermatozoa that were not fully 

 mature (those of a few days before ejaculation), some union of broken 

 ends does occur, inasmuch as divided treatments in this case did give 

 reduced frequencies of translocations. However, the interpretation 

 applied by Lane to the similar results in plant material (see Chap. 7, 

 Sect. 15, and the discussion) is not yet ruled out, namely, that an earlier 

 irradiation may in such cells decrease the susceptibility of the chromo- 

 somes to breakage by a later irradiation. It is possible, alternatively, in 

 view of Liining's (1952a, b, c, d) findings, that the stage at which the 

 undivided treatment was applied was an especially susceptible one. 



The question arises as to why the frequency of gross structural changes 

 produced by irradiation of mature spermatozoa was proportionate to 

 only the ^^ power instead of the square of the dose. Since the individual 

 breaks vary as the first power of the dose and most of the gross structural 

 changes studied must have involved two of these breaks, a square relation 

 might at first sight have been expected if the two breaks were usually 

 produced independently of one another. A main answer to this ques- 

 tion, in this material, lies in the fact that, at the higher doses used, a 

 considerable proportion of the structural changes involve more than two 

 breaks. Thus at the higher doses the number of breaks involved in the 

 structural changes is not adequately represented by a mere count of the 

 number of structural changes. Moreover and more important, the more 



