| Summary: | It has been known for many years that chilling at sensitive stages of embryonic or larval life profoundly affects the development of cold-blooded animals. For instance, the wing pattern of butterflies can be changed by cooling their pupae; the central European tortoiseshell, Vanessa urticae , treated in this way emerges in a form closely resembling the arctic species V. polaris. Drosophila and other insects cooled as larvae show bizarre characteristics, while sea-urchin and fish cooled as early embryos develop into monstrosities. Similar aberrations occur naturally as a result of genetic mutations and can be produced experimentally in these animals by X-rays and other physical and chemical agents applied at critical periods of development (see Ford, 1945; Goldschmidt, 1938). X-rays administered at critical stages in pregnancy may induce embryonic death or foetal deformities in mammals. Some of the monstrosities are resorbed, aborted, or stillborn; others survive birth (Russell, 1950, 1956; Russell & Russell, 1954 a , b Wilson, 1954).
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