Weiss's theory on the change of Schr?dinger wave functional on a surface as the surface changes is given in a complete form, allowing the Lagrangian of the field to contain all derivatives of the field quantities. The integrability of the resulting equation is proved by making use of the fact that the corresponding Hamilton-Jacobi equation is integrable. This gives at the same time a proof of the Lorentz invariancy of the commutation relations between the various conjugate variables, which so far remained obscure as soon as we allow derivatives higher than the second of the field quantities to appear in the Lagrangian.