THE functions of the sarcolemma of skeletal muscle are still uncertain, because of our lack of knowledge of its structure and detailed distribution over the entire surface of the muscle cell. Most of our knowledge is derived from electron microscope investigations and the sarcolemma is regarded as a semi-permeable unit membrane about 100 Å thick, which directly encloses the contents of the cell. This membrane is probably bimolecular and lipoprotein in character, and similar to that which is thought to invest most animal cells and many of their cytoplasmic organelles. Immediately external to the sarcolemma is a layer about 500 Å thick; this is an extracellular deposition of material of moderate density (the so-called basement membrane1) which gradually diminishes in density as it extends from the sarcolemma into extracellular space and merges with the ground substance of the endomysial connective tissue. Because of its extreme thinness, the sarcolemma is difficult to observe in its longitudinal section; on the other hand, its characteristic unit membrane structure can usually be recognized in transverse section and occasional thickenings and invaginations of the sarcolemma are known to occur1 at certain regions of the cell. However, using thin-section techniques it is not easy to follow the detailed variations in its course over large areas of the cell surface. For structural investigations of this type the surface replication methods, such as the layer-stripping method developed by Reed and Rudall, are probably far more useful, as the various levels exposed in the tissue may be examined over wide areas2. Early investigations using the layer-stripping method revealed the sarcolemma as a continuous elastic sheet, capable of accommodating itself to the structures it encloses, since it often bears undulations corresponding to the striations of the myofibrils3.