  We addressed the inconsistency between the electron mass anisotropy ratios
determined by the far-infrared experiments and DC conductivity measurements. By
eliminating possible sources of error and increasing the sensitivity and
resolution in the far-infrared reflectivity measurement on the single
crystalline and on the polycrystalline La1.84Sr0.16CuO4, we have unambiguously
identified that the source of the mass anisotropy problem is in the estimation
of the free electron density involved in the charge transport and
superconductivity. In this study we found that only 2.8 % of the total
doping-induced charge density is itinerant at optimal doping. Our result not
only resolves the mass anisotropy puzzle but also points to a novel electronic
structure formed by the rest of the electrons that sets the stage for the high
temperature superconductivity.
