1. The surface of the unfertilized sea urchin egg is folded and the folds are reversibly eliminated by exposing the egg to hypotonic sea water. If the plasma membrane is outside the layer of cortical granules, unfolding may explain why the membrane capacitance per unit area decreases (and does not increase) when a sea urchin egg is put into hypotonic sea water.
2. The degree of surface folding markedly increases after fertilization, which provides an explanation for the increase in membrane capacitance per unit area observed after fertilization.
3. The percentage reduction in membrane folding in fertilized eggs after immersion in hypotonic sea water is probably sufficient to explain the decrease in membrane capacitance per unit area observed in these conditions.