  We investigate the hole dynamics in two prototypical high temperature
superconducting systems: La$_{2-x}$Sr$_{x}$CuO$_{4}$ and YBa$_{2}$Cu$_{3}%
$O$_{y}$ using a combination of DC transport and infrared spectroscopy. By
exploring the effective spectral weight obtained with optics in conjunction
with DC Hall results we find that the transition to the Mott insulating state
in these systems is of the "vanishing carrier number" type since we observe no
substantial enhancement of the mass as one proceeds to undoped phases. Further,
the effective mass remains constant across the entire underdoped regime of the
phase diagram. We discuss the implications of these results for the
understanding of both transport phenomena and pairing mechanism in high-T$_{c}$
systems.
