  The primary goal of this paper is to make a direct comparison between the
measured and model-predicted abundances of He, C and N in a sample of 35
well-observed Galactic planetary nebulae (PN). All observations, data
reductions, and abundance determinations were performed in house to ensure
maximum homogeneity. Progenitor star masses (M < 4M_sun) were inferred using
two published sets of post-AGB model tracks and L and T_eff values. We conclude
the following: 1) the mean values of N/O across the progenitor mass range
exceeds the solar value, indicating significant N enrichment in the majority of
our objects; 2) the onset of hot bottom burning appears to begin around 2 solar
masses, i.e., lower than ~5 M_sun implied by theory; 3) most of our objects
show a clear He enrichment, as expected from dredge-up episodes; 4) the average
sample C/O value is 1.23, consistent with the effects of third dredge-up; and
5) model grids used to compare to observations successfully span the
distribution over metallicity space of all C/O and many He/H data points but
mostly fail to do so in the case of N/O. The evident enrichment of N in PN and
the general discrepancy between the observed and model-predicted N/O abundance
ratios signal the need for extra-mixing as an effect of rotation and/or
thermohaline mixing in the models. The unexpectedly high N enrichment that is
implied here for low mass stars, if confirmed, will likely impact our
conclusions about the source of N in the Universe.
