  We study gravitational instabilities in disks, with special attention to the
most massive clumps that form because they are expected to be the progenitors
of globular-type clusters. The maximum unstable mass is set by rotation and
depends only on the surface density and orbital frequency of the disk. We
propose that the formation of massive clusters is related to this largest scale
in galaxies not stabilized by rotation. Using data from the literature, we
predict that globular-like clusters can form in nuclear starburst disks and
protogalactic disks but not in typical spiral galaxies, in agreement with
observations.
