  Here we model a star forming factory in which the continuous creation of
stars results in a highly concentrated, massive (globular cluster-like) stellar
system. We show that under very general conditions a large-scale gravitational
instability in the ISM, which triggers the collapse of a massive cloud, leads
with the aid of a spontaneous first generation of massive stars, to a standing,
small-radius, cold and dense shell. Eventually, as more of the collapsing
matter is processed and incorporated, the shell becomes gravitationally
unstable and begins to fragment, allowing the formation of new stars, while
keeping its location. This is due to a detailed balance established between the
ram pressure from the collapsing cloud which, together with the gravitational
force exerted on the shell by the forming cluster, acts against the mechanical
energy deposited by the collection of new stars. We analyze the mass spectrum
of fragments that result from the continuous fragmentation of the standing
shell and show that its shape is well approximated at the high mass end by a
power law with slope -2.25, very close to the value that fits the universal
IMF. The self-contamination resultant from the continuous generation of stars
is shown to lead to a large metal spread in massive ($\sim$ 10$^6$ M$_\odot$)
clusters, while clusters with a mass similar to 10$^5$ M$_\odot$ or smaller,
simply reflect the initial metalicity of the collapsing cloud. This is in good
agreement with the data available for globular clusters in the Galaxy.
