The first components of the sheath to develop are two longitudinal columns, characterized by alternating light and dark bands. The columns, which lie on opposite sides of the future principal piece, are initially associated with the axial filament complex but soon make contact with the plasma membrane. Subsequently a layer of moderately dense material grows outwards from the lateral aspects of each column. The outgrowths lie just beneath the tail plasma membrane and contain evenly spaced filaments which are connected with the dark bands of the columns. The outgrowths from corresponding sides of each column eventually meet each other and the filaments they contain join end-to-end. Some parts of the sheath, separated from the plasma membrane by an expansion of the cytoplasm of the intraspermatid tail, become invested by membrane bound vacuoles. The filaments form into groups, and filaments within groups converge to produce the anlagen of the ribs of the mature sheath. The filaments lose their identity in these anlagen, which, like the columns, develop much finer filamentous structures. The ribs, and subsequently the columns, lose contact with the tail plasma membrane. The mature sheath, the ultrastructure of which is described in detail, is developed by further modification of the rib anlagen and longitudinal columns.