[en] The longitudinal mode in the Heisenberg model on a bilayer square lattice is studied using series expansion methods. It is demonstrated numerically that the longitudinal mode becomes degenerate with the magnon modes at the quantum phase transition between Néel and dimerized phases, thus forming a spin-1 multiplet in accord with the restoration of SU(2) symmetry. It is also shown that the magnon mode becomes degenerate with the triplet mode in the dimerized phase at the critical point, showing continuity of the excitation spectrum across the critical point.