The Effects on Foetal Development of Freezing Pregnant Hamsters

I T 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). Various hormones, drugs, toxic substances, and virus infections cause similar abnormalities (Gillman, Gilbert, & Gillman, 1948; Thiersch, 1952; Nelson, Asling, & Evans, 1952; Hicks, 1953, 1954; Giroud, 1954). By contrast, comparatively little is known about the effects of cold on early mammalian development. Vidovic (1952, 1956) found that rats at all stages of pregnancy survived cooling to deep body temperatures between 15° and 20° C. and occasionally as low as 12° C. The majority gave birth to normal young. The sensitivity to hypothermia increased after the 14th day and particularly after the 16th day of pregnancy so that litter size was small and the proportion stillborn increased. One rat cooled on the 17th day of pregnancy gave birth to three deformed foetuses. Courrier & Marois (1953, 1954), using a slightly different technique, found that uterine haemorrhage occurred in rats cooled to body temperature between 16° and 20° C. on the 12th to the 18th day of pregnancy. The