  Two-dimensional simulations of mixing and fall back in non-rotating massive
stars have been carried out using realistic initial models for the presupernova
star and assuming standard spherically symmetric explosions of 1.2*10^51ergs.
Stars of 15 and 25 solar masses with both primordial and solar composition were
modeled. The zero metallicity supernova progenitors were compact blue stars and
the amount of Rayleigh-Taylor induced mixing in them was greatly reduced
compared with what was seen in the red supergiants with solar metallicity. The
compact zero-metal stars also experienced more fallback than their solar
metallicity counterparts. As a result, the ejected nucleosynthesis from the two
populations was very different. For the simple explosion model assumed, low
metallicity stars ejected ejected too little iron and intermediate mass
elements even to explain the abundance patterns in the most iron-poor stars
found to date, suggesting that some important ingredient is missing. Rotation
is likely to alter these conclusions by producing a greater fraction of red
supergiants among Population III stars. The velocities of the heavy elements in
all models considered - both red and blue supergiants - were less than observed
in SN 1987A, suggesting that at least occasionally, asymmetric aspects of the
explosion mechanism and fallback play a major role in mixing.
