  We present simulations of a hard disc system and analyze the time evolution
of the dynamic heterogeneities. We characterize the time evolution of slow
regions and slow particles individually. The motion of slow clusters turns out
to be very restricted, i.e. a cluster is generated and annihilated in a spatial
region four times the size of its maximum extent. The residual motion of the
cluster can be traced back to subdiffusive motion of the constituent particles
and the process of absorption and loss of adjacent particles. The subdiffusive
dynamics is independent of how long the particles remain slow. Clusters of fast
particles show an even smaller reach, which seems to be due to their short life
time.
