  We perform a detailed analysis of the irreversible magnetization data of
Salem-Sugui et al. and Babis et al. of underdoped and optimally doped
YBa2Cu3O7+x single crystals. Near the zero field transition temperature we
observe extended consistency with the properties of the 3D-xy universality
class, even though the attained critical regime is limited by an inhomogeneity
induced finite size effect. Nevertheless, as Tc falls from 93.5 to 41.5 K the
critical amplitude of the in-plane correlation length gsiab0, the anisotropy
amma=gsiab0/gsic0 and the critical amplitude of the in-plane penetration depth
Lamdaab0 increase substantially, while the critical amplitude of the c-axis
correlation length gsic0 does not change much. As a consequence, the
correlation volume Vcorr increases and the ritical amplitude of the specific
heat singularity A decreases dramatically, while the rise of gsiab0 reflects
the behavior of the zero temperature counterpart. Conversely, although gsiab0
and lamdaab0 increase with reduced Tc, the Ginzburg-Landau parameter decreases
substantially and YBa2Cu3O7+x crosses over from an extreme to a weak type-II
superconductor.
