MANNER OF PRODUCTION OF MUTATIONS 555 



of "matrix," pull of centromere, synaptic forces exerted by homologous, 

 sister, and even heterologous chromosome threads, viscosity, protoplasmic 

 currents, temperature, vibrations, and gravitational, accelerational (cen- 

 trifugal), electric, and perhaps even magnetic pulls, etc. 



In view of all these influences acting on the speed and pattern of join- 

 ing of the broken ends, it is usually very difficult, at best, to draw safe 

 conclusions concerning the frequency of the primary breakages induced 

 by the application of radiation at different cell stages, and under differ- 

 ent accompanying conditions, from observations on the freciuencies with 

 which aberrations, such as acentric fragments, isochromosomes, and 

 exchange combinations, can be found after the lapse of several hours or 

 days following irradiation. In several articles Sparrow (e.g.. Sparrow and 

 Maldawer, 1950; Sparrow, 1951) has shown that differences in rejoining 

 are to a considerable degree responsible for the results obtained when 

 delayed observations of this sort are made. On the other hand, the 

 carrying out of immediate observations on breakage has usually been 

 impossible or impracticable since, until the chromosomes recondense, the 

 changes in them are seldom evident. Hence there is much uncertainty as 

 to the extent and the manner in which the susceptibility of the chromo- 

 somes to having primary breaks produced in them by radiation or other 

 mutagens varies with cell stage, chromosome state, and accompanying 

 conditions. In all cases the cjuestion must be asked, and can seldom be 

 answered: to what degree is a given difference caused by an effect on 

 joining rather than on breakage itself? When, however, the influence 

 in question can be proved to exert its effect only when applied during 

 irradiation, it can usually be concluded that that influence is on the 

 breakage process. 



15. INFLUENCE OF WATER AND OXYGEN 

 ON HIGH-ENERGY MUTAGENESIS 



Turning now from conditions known only in biological terms to those 

 of a physically and chemically defined nature which affect the production 

 of mutations (and possibly also chromosome breaks) by ionizing radi- 

 ation, it may be noted that the first such condition regarding which evi- 

 dence was obtained was that of desiccation versus a high water content. 

 Stadler's early observations on the far higher frequency of induced point 

 mutations in soaked sprouting seeds of barley than in dry dormant ones 

 had already pointed in this direction, but might have been caused by a 

 difference in developmental stage. Several later workers have confirmed 

 his observation on the effect of soaking seeds before irradiation in increas- 

 ing the frequency of X-ray-induced point mutations and have extended 

 the finding also to chromosome aberrations (e.g., Gustafsson, 1940, 1941; 

 Wertz, 1940; Gelin, 1941; Centner and Brown, 1941; Knapp and Kaplan, 



