The results obtained in the present work bring out clearly that the dried shell membrane is an open lattice-like network containing micropores of the order of 1 micron in mean equivalent diameter and whose surface density is roughly 20 millions per sq. cm. The gas transfer through this network is largely by viscous flow and accordingly does not obey Graham's law of diffusion. The data make clear that dried shell membranes are extremely permeable to all the gases examined and that wet membranes are completely impermeable. This fact leads one to suspect that during embryonic development, the rate of influx of gases is controlled in some measure by the moisture content of the shell membrane. The permeability technique developed here should be of value in the study of other membranes and especially natural membranes of biological importance.