[Autoradiographic study of the effect of temperature on cellular proliferation in late embryos ofPleurodeles waltlii Michah. (Amphibia, Urodela)].

We observed in Pleurodeles embryos, stage 34, that the duration of the cell cycle and its phases was approximately the same for every tissue but was easily modified by varying the temperature. The generation time and the duration of S phase in embryos submitted to a 12° C temperature instead of 26° C are tripled or quadrupled. A temperature rise produced a proportionale shortening inG 2 andM phases and a lengthening inG 1 phase. ThisG 1 phase is not detectable at 12° C but represent a 1/4 of the total generation time at 26° C. The more differentiated the cells are, the longer is theG 1 time. The cell population studied during these experiments are growing exponentially. Growth fraction, which represents the exponential growth basis, is temperature independent but has a tissue specificity. This growth fraction is smaller the more the tissue is differentiated. However, the relative rate of cell division, inversely proportional to the generation time, is temperature dependent and appears to control the embryo's relative rate of growth under different temperatures.