CHROMOSOME ABERRATIONS IN ANIMALS 655 



cedes recombination, or whether both phases are part of a single process 

 and occur simultaneously, could not be decided with certainty from the 

 data obtained in the earlier genetic experiments; but more recent evi- 

 dence, both genetical and cytological, is in accordance with the first of 

 these alternatives (reviewed in Bauer, 1939c; Muller, 1940). After the 

 breaks — or potential breaks — have been induced, the establishment of 

 new associations of ends is influenced by movement of the chromosomes. 

 In dividing cells such movement takes place as mitosis progresses, with 

 the result that breakage and recombination occur in close succession. 

 In the sperm head of Drosophila, the chromosomes are quiescent, so that 

 the types of rearrangement that depend on establishment of associations 

 among breakage ends at different loci are not realized until after the 

 spermatozoon has penetrated the egg in fertilization, when chromosome 

 movement incident to the formation of the male pronucleus presumably 

 provides opportunities for recombination (Muller, 1940; Kaufmann, 

 1941a; Dempster, 1941a; Makhijani, 1945). The delay between break- 

 age and recombination, which can be controlled by withholding the 

 treated males from copulation, permits experimental determination of 

 the time and method of combination of breakage ends in the production 

 of detectable aberrations. 



3-1. THE BREAKAGE PROCESS 



3-la. Induction of Breaks. Physical data indicate that ionizations are 

 localized along the paths of ionizing particles throughout the cells of an 

 irradiated tissue. The sequence of events that transpires between 

 ionization and the production of chromosomal lesions remains largely 

 conjectural (see Zirkle, 1949), although an understanding of possible 

 intermediate steps is slowly emerging. It is now generally agreed that 

 ionization produces chemical changes in the irradiated tissue (see, for 

 example, the discussion of Allsopp, 1948; Allsopp and Catcheside, 1948). 

 Available evidence suggests that chemical changes resulting from ioniza- 

 tion of materials within the chromosome itself may initiate the series of 

 reactions that lead to its breakage. Whether the chemical reactions 

 originate for the most part in molecules of constituent nucleic acids and 

 proteins, or are mediated through associated aqueous solutions, remains 

 to be determined. For this reason the process of induced chromosome 

 breakage cannot at present be described as exclusively direct or indirect 

 according to the definition formulated by Bacq (1951), which restricts the 

 direct effects to changes initiated by ionizations occurring in or on the 

 surface of organic molecules. Studies conducted in the past few years 

 showing the dependence of the yield of chromosomal aberrations on the 

 physiological conditions of the cell or organism speak strongly in favor of 

 the indirect reaction. Thus Baker and Sgourakis (1950) have shown 

 with Drosophila that the frequency of dominant and recessive lethals is 



