The hypothetical questions which were asked earlier may now be answered as follows. Myelin lipids do possess characteristics which could lead to a tightly organized, closely packed, highly stable membrane structure, while lipids in adjacent neurons and glial cells do possess characteristics which could give rise to a more loosely organized, less stable membrane structure. The greater degree of metabolic inertness of myelin lipids may be explained on this basis. The stability of myelin itself may also be based largely on the intermolecular cohesion between lipid molecules, since the lipid content of myelin is extraordinarily high. Physiological and clinical studies tend to support the concept that a surfeit of polyunsaturated lipids or a deficiency of long-chain sphingolipids can result in a more easily disrupted membrane. The most important prediction to be drawn from these speculations is that myelin may be rendered unstable in disease if its lipid composition is altered so that higher proportions of lipids containing polyunsaturated fatty acids and lower proportions of lipids containing very-long-chain fatty acids are present.