In sections of KMnO4-fixed, developing mouse sciatic nerves, the central gap of mesaxons in myelinating fibers is normally closed with close apposition of the outside ∼20 A dense strata of the two ∼75 A Schwann cell membranes. The two combined outside strata make the intraperiod line bisecting each myelin lamella. The ∼150 A mesaxon is elaborated spirally around the axon in either a right hand or left hand spiral, and its inside (cytoplasmic) ∼20 A strata in apposition form the major dense lines of myelin. In hypotonic solutions the lamellae of adult frog sciatic myelinated fibers split apart along the outside membrane strata apposed at the intraperiod line throughout the spiral. Under similar conditions the inside (cytoplasmic) strata of the membranes, in apposition at the major dense lines, do not separate. The ∼150 A membranous structure resulting from this is called an "internal compound membrane." The double membranes of normal and control frog sciatic unmyelinated fibers have a central gap ∼100 to 150 A wide. After soaking in 4 to 10 times normal strength Ringer solution or 10 N sucrose-Ringer solution, this gap closes and a membranous structure ∼150 A wide resembling developing mouse mesaxons results. This is designated by the term "external compound membrane." The latter membranes resemble internal compound membranes, but their central dense zones, each consisting of two apposed outside membrane strata, are less dense.