The contributions of direct and indirect effects to biological damage were discussed. The direct effect in polythene and dry bovine serum is influenced by oxygen or temperature but not by small amounts of water. In contrast, the radiosensitivity of enzymes in wet yeast cells is 2–50 times higher than in dry cells, suggesting a high contribution from the indirect effect. The respiratory system of mouse-liver slices is affected by doses higher than 10⁶ r, with sensitivity determined by cytochrome c, destroyed by indirect action. Yoshida ascites cells show a drop in respiration and glycolysis after irradiation, with changes in ATP levels. Incorporation of dl-phenylalanine-¹⁴C into mouse liver cells is higher after irradiation. The rate of phosphorus-32 incorporation into rat thymus DNA is reduced after irradiation. Increased tracer incorporation may be due to reduced body pool size. A single sub-sterilizing dose of 1,050 r to rabbit ovaries is less damaging than fractionated doses. The sterilizing dose for mice is not dependent on dose-rate. In Drosophila, X-rays can induce true reversals of the forked mutation. Alkylating agents produce point mutations in loci not mutated by X-rays. Chromosomes of spermatids are more sensitive to X-ray-induced mutations than mature sperm. Formaldehyde induces crossing-over in spermatogonia. X-ray-induced chromosome breaks may recover if sperm are retained. X-rays induce more small-effect mutations than recessive lethals in Drosophila. Irradiated chromosomes reduce fly viability and development. Two films showed the effect of α-ray microbeams on mitotic behavior. Sensitivity increases from prophase to metaphase. Chromosome irradiation is more effective in causing anaphase and telophase abnormalities. High X-ray doses in Vicia faba cause rapid-rejoining chromosome breaks. A session on radiation mutagenesis in plant breeding was held. Radiation-induced mutations in soybeans have limited economic value. Mutated shoots in black currants show promise. A review of Swedish work on directed mutations in barley was presented. An assemblage on population genetics was held, discussing the stability of development. Dr. Thoday highlighted the genetic diversity in out-breeding species and the role of heterozygosity in adaptation to environmental changes. Environmental heterogeneity was classified as random, directional, or cyclic. Cyclic conditions may lead to heterozygous adaptation. Dr. Thoday suggested that heterozygosity helps populations adapt to environmental cycles. Papers discussed the ability of individuals to maintain normal development despite disturbances. Mr. Beardmore reported on the effects of temperature cycles on stability in Drosophila. Mr. Maynard Smith discussed developmental flexibility in Drosophila, showing that outbreds can acclimate better to high temperatures. Dr. Rees discussed the stability ofThe contributions of direct and indirect effects to biological damage were discussed. The direct effect in polythene and dry bovine serum is influenced by oxygen or temperature but not by small amounts of water. In contrast, the radiosensitivity of enzymes in wet yeast cells is 2–50 times higher than in dry cells, suggesting a high contribution from the indirect effect. The respiratory system of mouse-liver slices is affected by doses higher than 10⁶ r, with sensitivity determined by cytochrome c, destroyed by indirect action. Yoshida ascites cells show a drop in respiration and glycolysis after irradiation, with changes in ATP levels. Incorporation of dl-phenylalanine-¹⁴C into mouse liver cells is higher after irradiation. The rate of phosphorus-32 incorporation into rat thymus DNA is reduced after irradiation. Increased tracer incorporation may be due to reduced body pool size. A single sub-sterilizing dose of 1,050 r to rabbit ovaries is less damaging than fractionated doses. The sterilizing dose for mice is not dependent on dose-rate. In Drosophila, X-rays can induce true reversals of the forked mutation. Alkylating agents produce point mutations in loci not mutated by X-rays. Chromosomes of spermatids are more sensitive to X-ray-induced mutations than mature sperm. Formaldehyde induces crossing-over in spermatogonia. X-ray-induced chromosome breaks may recover if sperm are retained. X-rays induce more small-effect mutations than recessive lethals in Drosophila. Irradiated chromosomes reduce fly viability and development. Two films showed the effect of α-ray microbeams on mitotic behavior. Sensitivity increases from prophase to metaphase. Chromosome irradiation is more effective in causing anaphase and telophase abnormalities. High X-ray doses in Vicia faba cause rapid-rejoining chromosome breaks. A session on radiation mutagenesis in plant breeding was held. Radiation-induced mutations in soybeans have limited economic value. Mutated shoots in black currants show promise. A review of Swedish work on directed mutations in barley was presented. An assemblage on population genetics was held, discussing the stability of development. Dr. Thoday highlighted the genetic diversity in out-breeding species and the role of heterozygosity in adaptation to environmental changes. Environmental heterogeneity was classified as random, directional, or cyclic. Cyclic conditions may lead to heterozygous adaptation. Dr. Thoday suggested that heterozygosity helps populations adapt to environmental cycles. Papers discussed the ability of individuals to maintain normal development despite disturbances. Mr. Beardmore reported on the effects of temperature cycles on stability in Drosophila. Mr. Maynard Smith discussed developmental flexibility in Drosophila, showing that outbreds can acclimate better to high temperatures. Dr. Rees discussed the stability of