The experiments of Pfeiffer ( '91) and Vl6s ( '26) gave the surface forces of living cells between 10 and 20 dynes per centimeter, which might conceivably be due to interfacial tension. I n order to distinguish between an interfacial and a membrane tension, it is necessary to measure the tension as a function of the surface of the cell. An interfacial tension should be independent of the area of the interface, while that of a membrane should increase as the surface increases.
The flattened, or 'sessile,' drop method used by VlQs is not applicable-except with the microscope centrifuge-f or many eggs, including the Arbacia, because they do not flatten measurably under gravity. I n a preliminary experiment in the summer of 1930, it was found that at least ten unfertilized Arbacia eggs were required to support a bit of cover-slip that weighed about a milligram in water. This showed that it was necessary to measure forces at least as small as a microgram weight under sea-water. Let us assume that only tangential surface forces are present and that any rigidity and viscous effects of the surface and interior may be neglected. If, at any point, P is the pressure difference across the surface, T is the surface force, and R, and R, are the principal radii of curvature, they should satisfy the classical equation-1