Having repeated some older experiments, Brachet (1949) has shown that amphibian gastrulae submitted to a heat shock either stop their development or exhibit numerous specific malformations. Cytochemical and biochemical analyses made by several workers have left little doubt that RNA is particularly sensitive to heating (Brachet, 1949; Mookerjee, 1953; Hasegawa, 1955) Furthermore, it has recently been shown (Rosenbaum, 1960) that in amphibian eggs blocked at gastrulation by exposure to very high temperatures, catheptic activity increases.

As far as mammals are concerned, high temperatures have been applied to pregnant females but not directly to the embryos, so that the results cannot be easily compared with those obtained in other classes of vertebrates. Several authors have succeeded in obtaining resorption, abortion or rarely slight abnormalities (Hsu, 1948; MacFarlane et al., 1957; Brinsmade & Rübsaamen, 1957; Svetlov & Korsakova, 1959) which differ greatly from those obtained in amphibians.

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