  The general metric for N-dimensional spherically symmetric and conformally
flat spacetimes is given, and all the homogeneous and isotropic solutions for a
perfect fluid with the equation of state $p = \alpha \rho$ are found. These
solutions are then used to model the gravitational collapse of a compact ball.
It is found that when the collapse has continuous self-similarity, the
formation of black holes always starts with zero mass, and when the collapse
has no such a symmetry, the formation of black holes always starts with a mass
gap.
