Permeability of the Fundulus Egg to Ions

Abstract

In previous studies on the permeability of the Fundulus egg, little attention has been paid to the fact that the embryo is enclosed by 2 membranes. One of these is the skin of the embryo itself; or, at very early stages before the blastodisc has completely enclosed the yolk, the delicate vitelline membrane which covers the yolk. The other is the chorion, the noncellular, flexible, inelastic outer shell, which has no organic connection with the embryo, but retains a cushioning fluid layer about it. Some of the criteria of permeability used in the previous work on Fundulus eggs, such as cessation of the heartbeat, and coagulation of the embryo, by acid penetration, may have required that both membranes be traversed by the substance studied. Others, of which Loeb's specific gravity test is a type, probably involved only the chorion. But in none of the earlier papers have data been presented for a satisfactory comparison of the permeabilities of the 2 membranes.

It was thought that evidence bearing on this question might be obtained by a method which Michaelis has recently applied to the study of the permeability of certain non-living membranes, such as dried collodion. Michaelis has described the permeability to ions of these membranes in terms of concentration potentials measured across the membranes between solutions of N/10 and N/100 KCl. This general method with certain modifications has been applied to the Fundulus egg material by measuring E.m.f. across the membranes of a single egg from inside to outside. The egg was impaled on a capillary pipette which, in connection with a calomel half cell made up in sea water, constituted the inside electrode; and was immersed in a solution into which dipped the outside electrode, of similar construction.