  We have demonstrated recently that RHIC heavy ion data concerning particle
production at small and intermediate transverse momenta become very
transparent, if one realizes that there are two sources of particles
production: the corona -- due to the interactions of the peripheral nucleons of
either nucleus, and the core -- representing the high density central zone. We
extend our analysis to SPS heavy ion data. Again, we find that the nontrivial
centrality dependence of particle production (being stronger than at RHIC) is
simply due to an increasing corona contribution with decreasing centrality. The
core contribution shows no centrality dependence (concerning particle ratios),
and can be described in exactly the same way as at RHIC, with one exception:
there is somewhat less collective radial flow at SPS compared to RHIC. Apart of
this (and a trivial volume effect), the core portions produced in different
colliding systems at different energies (RHIC, SPS) look the same.
