At similar temps., the eggs of Rana temporaria developed most rapidly, which is correlated with its more northerly distr., its greatest restriction in the south, its extension to the highest altitudes and its earliest spring breeding. Its spawn floats in water, is not deposited among weeds, and the animal half of the egg is very dark brown in color, all of which are adjustments to utilize solar heat. The eggs of R. esculenta developed least rapidly. The sp. is southern in distr. and is a later spring breeder. Its eggs do not float and are colored light brown, better to reflect heat. Bufo bufo is intermediate in the rate at which its eggs develop, in distr. and in time of breeding. Its eggs do not float but the animal hemisphere is darkly pigmented.