The double nature of the intermediate dense line in peripheral nerve myelin

In myelin of peripheral nerves it is shown that the intermediate dense line is split after aldehyde fixation (with uranium post‐treatment) with a 20 Å gap intervening between the apposed outer leaflets of the Schwann cell membrane. This gap is permeable to colloidal lanthanum, which indicates that it is continuous with the extracellular space. Our observations are discussed in light of data of other investigators.