Sciatic nerves of chick embryos, 12 to 18 days incubation, were examined in freeze-fracture replicas with special emphasis placed on the development of tight junctional contacts in the myelin sheaths. In stages of beginning myelination short isolated particulate chains (focal tight junctions) appear in fracture faces of the adjacent membranes in the outer myelin lamellae, i.e., the outer mesaxon. In stages of progressing myelination these tight junctional elements elongate and become more numerous. They can also be found in the membranes of the inner mesaxons, the paranodal loops and the intramyelinic cytoplasmic inclusions. In fibers of advanced myelinogenesis a fusion of these isolated tight junctions--either end-to-end or at an angle--gives rise to continuous zonulae occludentes. This contact zone extends in the mesaxonal membranes along the direction of the fiber, whereas in paranodal myelin it acquires a helical course joining the membranes of the paranodal loops. It is proposed that this zonula occludens, which seals the cytoplasmic border of the Schwann cell, separates an intramyelinic from an extramyelinic, extracellular space already during the developmental stages of myelinogenesis.