CHROMOSOME ABERRATIONS IN ANIMALS 661 



1941b, 1948b). One of these, reported by Demerec and Sutton (1940) 

 involved a change in the Notch region of the X chromosome of D. 

 melanogaster. Cytogenetic analysis showed that a deficiency in one 

 chromatid extended from 3C8 to 3E5, inclusive, whereas in the other the 

 deficiency extended from 3C7 to 3E5. Thus the two breaks to the right 

 occurred at the same point, the two to the left at different points although 

 very close together. It seems probable that the breaks in 3C7 and 3C8 

 were caused by the passage of a single ionizing particle but that the spread 

 of the effect was greater along one chromatid than along the other. 



Such observations on chromosome rearrangements, and a comparable 

 series on gene mutations in Drosophila, are difficult to reconcile with the 

 theory that the chromosomes of the spermatozoon are unsplit, as some 

 investigators have assumed (e.g., Muller, 1940, 1941). Treatment of 

 Drosophila spermatozoa with chemical agents, such as nitrogen mustard, 

 produces types and proportions of lethals and chromosomal rearrange- 

 ments that can be explained most readily on the assumption that to a 

 large extent they originate as mosaics (Kaufmann, Gay, and Rothberg, 

 1949). Perhaps the specific action of this type of agent will provide the 

 precise information that X-ray treatment does not provide concerning 

 the number of experimentally separable strands that comprise the 

 chromosomes of the spermatozoon of Drosophila. 



In some materials lesions or constrictions are seen at anaphase in 

 chromosomes that have been irradiated at an earlier stage, and these have 

 been interpreted as half-chromatid breaks (e.g., in Carlson, 1938a, on 

 Chortophaga). Half-chromatid breaks have also been observed in 

 Tradescantia (Swanson, 1943, 1947), especially after treatment of the 

 cells with ultraviolet radiation. Since the sphere of action of an ultra- 

 violet quantum rarely encompasses more than one strand of a longi- 

 tudinally split chromosome, the half-chromatid breaks suggest that 

 under certain conditions radiations may act selectively on the com- 

 ponent units of a multiple-strand chromosome. A striking demonstra- 

 tion of such action has been reported by Slyzinski (1950), confirming an 

 earlier observation of Marshak (1936) that X-ray treatment of Dro- 

 sophila embryos induces structural rearrangements affecting only a few 

 of the chromonemata of the chromosomes that will form the giant struc- 

 tures seen in the salivary-gland cells. In these bodies the rearrangements 

 are discernible as deficiencies, inversions, and translocations, involving 

 in some cases less than one-sixteenth of the diameter of the chromosome 

 (Fig. 9-12). 



3-1 c. The Distribution of Breaks. Cytological examination of salivary- 

 gland chromosomes permits precise determination of the location of 

 radiation-induced breaks participating in viable recombinations. Exam- 

 ination of many hundreds of rearrangements has shown that the breaks 

 occur in the intervals between bands, suggesting that the integrity of the 



