The uptake and distribution of water in the embryo of Xenopus laevis (Daudin).

Water is the only constituent of the amphibian embryo that is taken u p from the environment in appreciable quantities dur ing the course of development, and at hatching it occupies nearly three t imes as m u c h space in the embryo as all the other consti tuents combined. I n spite of its small molecular size, however, and its ability to diffuse freely throughout the embryo, water is not uniformly distr ibuted nor is its distribution constant. Dur ing the early stages of development, large liquid-filled spaces with low solid content—the blastocoel and archenteron-—form between the cells, increasing and decreasing in volume as development proceeds; and although amphibian embryos normally develop in an environment which contains little or no solutes, they can tolerate a wide range of external osmotic pressures. Factors which control the uptake and distribution of water in the embryo may therefore be expected to play an important part in growth and morphogenesis. T h e experiments described in this paper were under taken to obtain quantitative information on which to base a study of these factors, in the embryo of Xenopus laevis (Daudin) . For this purpose it was necessary to measure changes in the water content of t h e embryo as a whole, its cells and intercellular cavities; bu t the difficulty of separating the water in the different compar tments makes direct measurement of water content impracticable. However, if it can be shown that , as in the explanted tail buds s tudied by Rappapor t (1954), all embryonic volume changes are the result of changes in water content , t hen it is merely necessary to measure the changing volume of the embryo and its compar tments to obtain the information required. T h e embryo and its cells have a very irregular shape which precludes the use of linear measurements for this purpose , b u t the volume can be calculated directly from the density of the embryo (p) and i ts weight in water, reduced weight ( R . W . )