  We show that all supersymmetric Type IIA D-branes can be constructed as bound
states of a certain number of unstable non-supersymmetric Type IIA D9-branes.
This string-theoretical construction demonstrates that D-brane charges in Type
IIA theory on spacetime manifold $X$ are classified by the higher K-theory
group $K^{-1}(X)$, as suggested recently by Witten. In particular, the system
of $N$ D0-branes can be obtained, for any $N$, in terms of sixteen Type IIA
D9-branes. This suggests that the dynamics of Matrix theory is contained in the
physics of magnetic vortices on the worldvolume of sixteen unstable D9-branes,
described at low energies by a U(16) gauge theory.
