Surrounding the myelin sheath, there is a thin membrane called neurilemmal sheath. This is also called neurilemma or sheath of Schwann. This contains Schwann cells, which have flattened and elongated nuclei. The cytoplasm is thin and modified to form the thin sheath of neurilemma enclosing the myelin sheath.One nucleus is present in each internode of the axon. The nucleus is situated between myelin sheath adn neurilemma.At the node of Ranvier (where myelin sheath is absent), the neurilemma invaginates and runs up to axolemma in the form of a finger like process.In nonmyelinated nerve fiber, the neurilemma continuously surrounds axolemma. Neurilemma is absent in central nervous system. Neurilemma is necessary for the formation of myelin sheath (myelinogeneis).