  We analyze the mass distribution of stellar black holes derived from the
light and radial velocity curves of optical stars in close binary systems using
dynamical methods. The systematic errors inherent in this approach are
discussed. These are associated primarily with uncertainties in models for the
contribution from gaseous structures to the optical brightness of the systems
under consideration. The mass distribution is nearly flat in the range
4-15M_sun. This is compared with the mass distribution for black holes in
massive close binaries, which can be manifest as ultraluminous X-ray sources
(L_x > 10^39 erg/s) observed in other galaxies. If the X-ray luminosities of
these objects correspond to the Eddington limit, the black-hole mass
distribution should be described by a power law, which is incompatible with the
flat shape derived dynamically from observations of close binaries in our
Galaxy. One possible explanation of this discrepancy is the rapid evaporation
of stellar-mass black holes predicted in recent multi-dimensional models of
gravity. This hypothesis can be verifed by measuring the stellar black-hole
mass spectrum or finding isolated or binary black holes with masses below
3M_sun.
