  Under the six-dimensional heterotic/type $IIA$ duality map, a solitonic
membrane solution of heterotic string theory transforms into a singular
solution of type $IIA$ theory, and should therefore be interpreted as a
fundamental membrane in the latter theory. This finding pointed to a gap in the
formulation of string theory that was subsequently filled by the discovery of
the role of $D$-branes as the carriers of Ramond-Ramond charge in type $II$
string theory. The roles of compactified eleven-dimensional membranes and
fivebranes in five-dimensional string theory are also discussed.
