  The origin of the asymmetry in the fragment mass distribution of low-energy
nuclear fission is considered from the semiclassical point of view. Using the
semiclassical periodic-orbit theory, one can define and quantify the shell
effect associated with spatially localized nascent-fragment (prefragment) part
of the potential. We investigate the roles of prefragments in the deformed
shell effect using a simple cavity potential model but with realistic shape
degrees of freedom for describing the fission processes. The results suggest
that the prefragment magic numbers play essential roles in determining the
shapes at the fission saddles, which should have a close relation to the
fragment mass distribution.
