1. 1. Two membrane fractions were isolated from homogenates of the first stellar nerve of Dosidicus gigas by a sequence of differential, discontinuous and gradient centrifugations in sucrose solutions. Membrane Fraction I has an apparent density of 1.090 g/ml and membrane Fraction II a value of 1.140 g/ml. 2. 2. At low and high magnifications in the electron microscope, both fractions appear as rounded membrane profiles similar to the plasma membrane observed in the unfractionated tissue. No other subcellular component was observed. Fraction I has a thickness of 95-110 Å and that of Fraction II is 72-100 Å. 3. 3. The percentage lipid composition of the two membranes was established. The following lipid classes were tentatively identified: phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin, cholesterol, free fatty acids and the hydrocarbon n-pentacosane. The relative distribution of the fatty acids present in the phospholipids of the two membrane fractions was different. 4. 4. The morphological appearance, the yield of the two membranes and the distribution of (Na+-K+)-dependent, ouabain-sensitive ATPase has led us tentatively to relate Fraction I with the axolemma and Fraction II with the Schwann cell plasma membrane. © 1969.
