EFFECTS OF X RAYS ON THE DIFFERENTIATION OF GASTRULAS OF AMPHIBIANS AND ON THE DIFFERENTIATION OF THE FEATHER GERMS IN THE SKIN OF CHICKEN EMBRYOS M. REYSS-BRION Laboratoire d’Embryologie et de Tératologie duC.N.R.S. et du Collége de France ABSTRACT X-ray irradiations of the ectoderm of young gastrulas of amphibians and of the skin of chick embryos show that the tissues that are differentiating are very radiosensitive but that the inductor tissues are able to support very strong doses of X-rays. Autoradiographic and biochemical studies point out a fall in the DNA synthesis in the tissues after trradiation. I have irradiated in toto young gastrulas of an amphibian urodele, Pleuvodeles waltlii (Michah.), with various doses of N-rays. After a dose of 1170 R, the embryos are deprived of a nervous system in 95% of the cases, Higher X-ray doses kill all embryos. To find which anlage is affected by X-rays, I have irradiated either the reacting ectodermal field or the inductive chordomesodermal field with 1170 R (Fig. 1). Following irradiation of the ectoderm, the differentiation of the ectoderm into nervous organs is inhibited in 86% of the cases and remains incomplete in the others. Following irradiation of the chordomesoderm, the inductive potentialities of the dorsal blastoporal lip are active in 100% of the cases. This property is retained even after very strong doses of X-rays, e.g., 150,000 R (Fig. 2). These results indicate that at the gastrula stage even very strong doses of X-rays have no important effect on the inductor, while very low doses are able to suppress the differentiation of the competent field, In order to study this point further, I irradiated the ectodermal cap of young gastrulas with lower doses of X-rays (1050, 800, 300, and 150 R), 1001