244 C. P. SwANSON AND Bengt Kihlman 



has been carried out (Beatty, Beatty and Collins, 1956), 

 and the data are in conformity with expectations. 



The results discussed above deal generally with the rejoining 

 process, and shed little or no light on the breakage of chromo- 

 somes by radiations. Except for centrifugation and infrared 

 radiation, breakage appears to be unmodifiable by posttreat- 

 ments, and there is no experimental evidence which suggests 

 that breakage by radiation is mediated through the normal 

 metabolic pathways of the cell. It is of interest here to recall 

 that there is a lack of intensity effect in the mature sperm of 

 Drosophila (Muller, 1940) and in the generative cells of de- 

 hydrated pollen grains of Tradescantia (J. C. Kirby-Smith, 

 unpublished). These are cells in which the chromatin is 

 densely packed, and where it is unlikely that metabolic 

 systems having a high yield of free energy would be operative 

 to provide the necessary requirements for rejoining. Metabolic 

 activity in these cells would increase with the uptake of water 

 which occurs at the time of fertilization for Drosophila sperm 

 and during the germination of Tradescantia pollen, and the 

 opportunities for rejoining would become available. The fact 

 that a reduction in aberrations in these two types of cells is 

 obtained when irradiation is carried out in the relative absence 

 of oxygen is added support for the concept that breakage 

 itself is determined only by the physical and radiochemical 

 events in the Latar jet-Gray reaction chain rather than by 

 the succeeding metabolic events (see later). 



Before attempting to fit the above data into a model system 

 it will be well to consider, in a comparative way, the effects of 

 chemical mutagens on aberration induction. A large variety 

 of chemicals which differ appreciably in structure and reactiv- 

 ity can induce chromosomal aberrations, but it seems desirable 

 to confine our attention to some of those which have been 

 studied intensively. These are di(2: 3-epoxypropyl) ether 

 (DEPE), 8-ethoxycaffeine (EOC), and maleic hydrazide (MH). 

 Viciafaba root-tips have been used as experimental material. 

 It is impossible as yet to say how any of these chemicals affect 

 chromatin to bring about the induction of aberrations, but it 



