Cytologists and geneticists work with raciiatioiis for two reasons. First, tlie cytogeiietic effect of radiations is an extremely important factor in the total picture of radiation effects on organisms. Secoiid, it has seemed logical to hope that the study of radiation effects on tlic chroiiiosome-gene system would give important insight into thp 1iatur.c of that system. Radiation may be ayplied quantitatively nnd tlic cytogenetic effects measured quantitatively. The hope is that by comparing tlie cytogeiietic effects with effects on simpler systems, the iiicchaiiisms of chromosomc cliangcs, including mutation, may be deduced. From these iiicchanisms sonietliiiig necessarily should be learned about the cytogmetic systems themselves. TJiitil recent years a paradoxical situation has cxisted wherein much more mas known about cytogeiietic effects of radiations than about their effects on the supposedly simpler biochemical systems d i i c h should supply the a n s w rs to some of our problems. The paradox has centered arouiid the problem of radiation sensitivity. The biochemical systems that had been studied up until a few years ago could be ineasurably affected only by radiation doses very much higher than those required to produce cytogeiietic effects. It would there-' This work has been supported by a grant from the Coirimittc>e oil Groa tli, National Researell Coiinril, acting for tlic Anierirart C:inrer Society. Riucr tlie preseiitatiori of this paper, much of the material lies appeared in the Proceedings of the National Academy of Sciencev, 54 : 32d, 1948.
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