  Quantum Molecular Dynamics (QMD) calculations are used to study the expansion
phase in central collisions between heavy nuclei. The final state of such a
reaction can be understood as the result of a entropy conserving expansion
starting from a compact source. The properties of this hypothetic source,
however, are in conflict with the assumptions used in fireball models.
Moreover, this hypothetical source is not formed in the dynamical evolution of
the system.
