Salt and water regulation in the embryos of freshwater pulmonate molluscs. II. Sodium uptake during the development of Biomphalaria sudanica.

In the previous paper (Beadle, 1969 a) it was shown that the eggs of Biomphalaria sudanica and Lymnaea stagnalis are, from the moment of laying, immersed in the capsular fluid between which and the outside water there is a free and rapid exchange of water and inorganic ions through the capsular membrane and outer jelly. It is therefore to be expected that the egg and early embryo can regulate their salt and water content in a very hypotonic medium before any special cells are differentiated for this purpose. The object of the experiments here described was to test this expectation by studying the uptake of sodium by means of Na during the course of development, to determine the relation between uptake and the external concentration of sodium and to discover how far uptake is dependent upon oxidative metabolism. It would have been preferable to study also the uptake of potassium, but work with the short-lived isotope was impracticable in Kampala. From what is known about the inorganic composition of animal cells and body fluids it is likely that the sodium taken up by the developing embryo accumulates mainly in the extracellular spaces and cavities and that potassium is in higher concentration in the cells. But we have no means of deciding this directly and, since we cannot even roughly estimate the separate volumes of cells and spaces in a normal embryo, we are limited to determining, by means of ^Na, the total sodium in the whole embryo.