The complex changes that the Schwann cell undergoes during the process of myelination of peripheral nerve fibers have been observed and studied by many.  The initial envelopment of the axon occurs without interruption along the entire extent of the Schwann cell.  This process is sequenced by the in-folding of the Schwann cell surface so that a double membrane of the opposing faces of the in-folded Schwann cell surface is formed.  This membrane stretches and spirally wraps itself over and over as the in-folding of the Schwann cell surface continues.  As a result, the increase in the thickness of the extension of the myelin sheath in its cross-sectional diameter is easily ascertained.  It is also evident that each of the consecutive turns of the spiral increases in size along the length of the axon as the number of turns increase.  However, it is not clear whether or not the increase in length of the myelin sheath can be accounted solely by the increase in length of axon covered by each successive turn of the spiral, as previously explained.  
At the junction of two Schwann cells along an axon, the directions of the lamellar overhang of the myelin endings are of opposite sense. This junction, adjacent of the Schwann cells, constitutes the region designated as the node of Ranvier.