  We propose that massive central objects form in the centers of the bars which
must develop in young high-surface density galactic disks. Large-scale dynamics
shuts off the growth of the central mass before it reaches $\sim 2%$ of the
disk mass at the time, but this mass is sufficient to weaken the bar
substantially. Subsequent evolution of the galaxy can either complete the
destruction of the bar or cause it to recover, depending upon the angular
momentum distribution of later infalling material. We produce massive, fully
self-gravitating disks having roughly flat rotation curves which are quite
stable. If at least part of the central masses we require constitute the
engines of QSOs, then our picture naturally accounts for their redshift
dependence, since the fuel supply is shut off by the development of an inner
Lindblad resonance. A prediction is that massive objects should not be found in
halo dominated galaxies, such as low-luminosity, or low-surface brightness
galaxies.
