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In the past decade, planetary nebulae have assumed considerable importance in elucidating our understanding of the final stages of stellar evolution at low mass. This began with the work of Shklovsky, O’Dell and Seaton, who showed not only that the nuclei of these nebulae were among the hottest stellar objects, but also that they evolved on a track in the Hertzsprung-Russell diagram (the Harman-Seaton sequence) on a time scale very rapid by stellar evolutionary standards (~ 20,000 years).

SummaryThe distribution of kamacite, taenite, and plessite suggests that kamacite nucleated at the sulphides, which appear to have been plentifully dispersed in the parent taenite. Such a process could take place without the undercooling that is usually assumed in normal octahedrites. The plessite fields show internal boundaries that are probably relics of grain boundaries in the parent taenitesulphide aggregate. Microprobe investigations have been made of nickel distributions in and between kamacite, taenite, plessite, and schreibersite, and particular attention has been paid to the nickel distribution at the internal grain boundaries in the plessite. Evidence is presented for the diffusion of nickel along these boundaries and for their migration during the later stages of formation of the microstructure.

Mouse embryos were irradiated or sham-irradiated in utero during the pronuclear-zygote stage and examined 16 days later.A simple exponential equation closely approximates the relation between surviving fraction of embryos and absorbed dose up to the highest dose used (20 rads of neutrons, 100 rads of X-rays). This suggests (1) that in the dose-range studied, pronuclear-zygote inactivation was predominantly a single-event process (i.e. embryo death resulted from passage of one ionizing particle) with either radiation, and (2) that the relative biological effectiveness of the neutrons (approximately 4·5) was close to the low-dose limiting value. D0 was 19 rads for neutrons and 87 rads for X-rays.With increasing radiation dose, not only did prenatal mortality increase but the embryos also died earlier in development.Dose-fractionation experiments suggest that irradiation of the early pronuclear zygote impeded its progression to a less radiosensitive phase.

Mice immunized to Hymenolepis nana were exposed to 950, 1150, 1350 or 1450 R x-radiation, then injected with syngeneic marrow from non-immune donors. Other immunized mice were exposed to 700 R but were not given marrow. To test for persistence of immunity, the mice were challenged with H. nana eggs. Parasite larvae (cysticercoids) developed in only 3 (1 or 2 cysticercoids each) of 40 immunized-unirradiated control mice challenged with eggs. Cysticercoids developed on egg challenge in all unimmunized control mice.There was no difference in cysticercoid counts between immunized-un-irradiated controls and either the 700 R group or the mice exposed to 1150 R and given 40 × 106 marrow cells 1 day later. A small number of cysticercoids developed in most of the mice exposed to 1350 or 1450 R and given 40 × 106 marrow cells 1 day after irradiation. Cysticercoids also developed in most of a 950 R group given 0·5 × 106 marrow cells 1 day after irradiation and 40 × 106 cells 20 days later; the cysticercoid counts were not different from those in similarly immunized and irradiated mice given only 0·5 × 106 cells. Thus, no benefit was derived from the second injection of marrow (40 × 106 cells) delayed until 21 days after irradiation. In contrast, in immunized mice given 40 × 106 marrow cells 4 days after 950 R, cysticercoid counts were not different from those in immunized-unirradiated controls.The role of transplanted syngeneic marrow in promoting persistence of immunity acquired before irradiation is discussed.