Chinese hamster V79 cells and mouse EMT6 cells were made extremely hypoxic with a chamber technique that relies upon gas exchange rather than respiration-assisted oxygen depletion. Cells in these chambers were treated with heat (43degrees C) or heat plus x-irradiation, and colony-forming ability was determined. Hyperthermia: (a) kills exponentially growing V79 cells more efficiently than plateau-phase cells; (b) kills euoxic and extremely hypoxic cells equally; and (c) radiosensitizes euoxic and extremely hypoxic cells to the same degree, leaving the oxygen enhancement ratio unchanged.

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Amictic female Asplanchna sieboldi, reproducing asexually by diploid parthenogenesis, are predominantly saccate in form when fed paramecia. The addition of vitamin E to their diet causes their female offspring to develop regular outgrowths of the body wall, or humps. A continuous spectrum of intermediate forms connects the saccate (S) and fully humped ( H ) morphotypes. We have shown that this response can be studied quantitatively with the use of an ordinal scale for measuring the degree of hump development: five morphotypes are distinguished and assigned scores (S = 1; 11 = 2; E = 3; I3 = 4; H = 5 ) . The response of embryos to exogenous vitamin E, measured with this scale, is proportional to the amount of vitamin E in the diet of their saccate parents. When vitamin E is removed from the diet of humped females, offspring with humps continue to be produced for up to five generations, with a gradual decline in morphotypic score. This short-term inheritance of an environmentally-induced trait can be explained by the passive transmission from parent to daughter of a stable endogenous inducer, which may be vitamin E itself. However, the relationship between the magnitude of the response and the concentration of this hypothetical inducer is not a simple one. For example, when females are fed vitamin E for one generation, the mean score of the granddaughters is greater than that of their daughters, which received no additional exogenous vitamin E. The same considerations apply to the inheritance of increased nuclear number in certain organs of A. brightwelli, which is another response to increased levels of dietary vitamin E.Biologists in the late nineteenth century recognized that populations of a number of ecologically important freshwater organisms show drastic seasonal changes in morphology. Such changes, of sufficient magnitude to throw the taxonomy of some groups into a state of confusion, are now called cyclomorphosis (reviewed by Hutchinson, '67). Subsequent studies, especially on Daphnia and certain rotifers, have demonstrated seasonal changes in physiological characteristics as well; of particular interest are cases where organisms alternate between sexual and parthenogenetic asexual reproduction. These physiological changes are related to, but at least partially independent of, the morphological changes; for this reason, we are in the current series of papers using the more general term polymorphism to refer to all such variations in phenotype. The polymorphism J. EXP. ZOOL., 170: 157-168.under investigation here is not genetic; rather, the variations in phenotype generally occur within clonal populations and, where carefully investigated, are attributable to the action of specific environmental stimuli. The role of the geneotype is, in this context, viewed as secondary, in defining the qualitative and quantitative nature of the response to these stimuli. In a few cases, the polymorphic changes have been shown to be adaptive. In contrast to the familiar seasonal adaptive changes in vertebrates, the ...

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