  We give a semiclassical interpretation of the mass asymmetry in the fission
of heavy nuclei. Using only a few classical periodic orbits and a cavity model
for the nuclear mean field, we reproduce the onset of left-right asymmetric
shapes at the fission isomer minimum and the correct topology of the
deformation energy surface in the region of the outer fission barrier. We point
at the correspondence of the single-particle quantum states responsible for the
asymmetry with the leading classical orbits, both lying in similar equatorial
planes perpendicular to the symmetry axis of the system.
