Conformal symmetry on the world-sheet is used to derive the effective equations of motion of target space fields for the closed bosonic string up to the second massive level. These equations are calculated in the weal field approximation, but to all orders in the inverse Planck scale α′. Enlarged gauge symmetries of the target space fields are derived directly from both types of zero-norm states in the bosonic string spectrum. Some of these symmetries can be generated by world-sheet (1, 0) or (0, 1) primary field generators.