Evolution of deformation accross the main shell N=82-126*

We show that the deviations of the experimental moments of inertia from the rigid-body values at high spins, as well as deviations of the binding energies from the liquid-drop values, are due to shell effects. They are called ``shell moments of inertia'' and ``shell energies.'' The semiclassical Periodic Orbit Theory (POT) calculates these shell effects in terms of contributions from a few classical periodic orbits in the nuclear potential. The comparison of shell energies obtained in quantal calculations and in POT provides a new perspective on the occurrence of spherical magic numbers and of deformation in nuclei. In particular the evolution of the nuclear deformation with neutron number in the main shell N = 82-126 will be discussed.