Prior to the development of the uranium chain reaction and the possibility of obtaining large quantities of artificially produced radioactive isotopes as a direct or indirect result of this reaction, direct experimentation on the biological effects of beta rays were of necessity limited by the availability of pure beta-emitting substances. It has long been assumed, however, that beta rays would produce in biological systems effects identical with those produced by x-rays and gamma rays, since the latter radiations expend their energy by the ejection of high-speed electrons from the atoms of the biological material in which they are absorbed. The validity of this assumption has recently been indicated by the writer and others at Clinton Laboratories in two biological materials which were small compared to the range of beta particles: fern spores and eggs of the fruit fly, Drosophila melanogaster. With larger biological objects, however, the distribution of energy absorption differs markedly between beta ra...