XVI.—The Problem of Chromosome Re-arrangements in Somatic Cells ofDrosophila Melanogaster

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Abstract
The mechanism by which irradiation produces gross re-arrangements of chromosomes (inversions, translocations, deletions) is now generally believed to occur in two steps: (a) breakage of one or more chromosomes, yielding two or more points of breakage; and (b) reunion of broken ends in a novel combination. Thus the frequency with which a given treatment produces re-arrangements depends not only on the number of breaks produced, but on the circumstances under which reunion takes place. In Drosophila the fertilized ovum seems to offer particularly favourable conditions for breaks produced in the mature spermatozoon to join up into new combinations. In immature germ-cells, on the other hand, re-arrangements are much less readily produced by irradiation (Muller, 1930; Shapiro and Neuhaus, 1933; Shapiro, 1937; Glass, 1939). Evidence for the occurrence of spontaneous or induced re-arrangements in somatic cells of Drosophila is very scanty.
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