The isothermal gravitational collapse and fragmentation of a region wi
thin a molecular cloud and the subsequent formation of a protostellar
cluster are investigated numerically. The clump mass spectrum that for
ms during the fragmentation phase can be well approximated by a power-
law distribution dN/dM proportional to M-1.5. In contrast, the mass sp
ectrum of protostellar cores that form in the centers of Jeans-unstabl
e clumps and that evolve through accretion and N-body interactions is
described by a lognormal distribution with a width that is in excellen
t agreement with observations of multiple stellar systems.