which underlies the wave equation and which "symmetrizes" the loop laws for electric and magnetic fields. This was the "missing link" between the previously known laws of electromagnetism (Ampere's law, Gauss's law, and Faraday's law) and the full theory given in Maxwell's equations.
This is true, but the other major achievement of Maxwell's equations was his formulation of the displacement current:
https://en.wikipedia.org/wiki/Displacement_current
which underlies the wave equation and which "symmetrizes" the loop laws for electric and magnetic fields. This was the "missing link" between the previously known laws of electromagnetism (Ampere's law, Gauss's law, and Faraday's law) and the full theory given in Maxwell's equations.