Changes in Body Proportions Produced in Frog Embryos by Supra-Normal Temperatures

Several investigators have produced changes in body proportions by varying the conditions of temperature in which development occurs. Hoadley 1 has found that constant exposure to slightly supra-maximal temperatures produced gradations of microcephaly in Rana pipiens and Rana sylvatica larvae. Huxley 2 and his associates 3 and Gilchrist 4 have found that temperature gradients, within the ranges of temperatures normal to development, when properly applied to the developing amphibian egg for short intervals have a stimulative effect which is apparent in the development of the larva. Considering the fluid environment of the egg membranes and of the embryo itself, it is improbable that an embryo in nature would be subjected to a temperature gradient. However, short exposures of the whole egg to temperatures higher than the usual maximum for normal development must frequently occur. Possible long range effects on the developing embryo of abnormal temperatures due to the heating of the spawning ponds of certain amphibia and to fever in the case of the pregnant mammal present problems in this connection. The present work was undertaken to test the effect on body proportions of short exposures of the whole amphibian egg and embryo to temperatures above the normal.

The eggs of Hyla regilla, the Pacific tree frog, were used in this work. An effort was made to obtain the larger clusters of 25 or 30 eggs each, for the eggs of a single cluster were the basis for each experimental series in order that all individuals of each series might be as nearly alike in age and genetic constitution as possible. Each cluster was divided into 5 equal groups. When the eggs of Group I reached the late blastula stage, they were exposed to a higher temperature in a thermostatically controlled water bath. After the exposure they were returned to room temperature.