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In analyzing the low-angle X-ray diffraction patterns recorded from the normal and swollen my&n in frog sciatic nerve, a precise value is found for the centerto-center distance between the pair of membranes constituting the repeating structure in the myelin. The previously reported large increase in period when the nerve is put in hypotonic Ringer's solution is shown to be accompanied by a small decrease in the center-to-center distance of the membranes within one pair. Thus both a large amplitude swelling at the extracellular interface between membrane pairs and a moderate one at the cytoplasmic interface within the pair are observed. The two kinds of swelling-cytoplasmic and extracellular-are suficient to demonstrate crystallographically the correctness of the bilayer profile for a single myelin membrane. The existence of a lipid bilayer within the membrane iv confirmed by the scale of absolute electron density derived from the swelling experiments, and hydrated surface layers of protein are postulated.

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