  The low emittance and small momentum spread of the proton and deuteron beams
of the Cooler Synchrotron COSY combined with the high mass resolution of the
COSY-11 detection system permit to study the creation of mesons in the
nucleon-nucleon interaction down to the fraction of MeV with respect to the
kinematical threshold. At such small excess energies, the ejectiles possess low
relative momenta and are predominantly produced with the relative angular
momentum equal to zero. Taking advantage of these conditions we have performed
investigations aiming to determine the mechanism of the production of eta and
eta' mesons in the collision of hadrons as well as the hadronic interaction of
these mesons with nucleons and nuclei. In this proceedings we address the
ongoing studies of the spin and isospin dependence for the production of the
eta and eta' mesons in free and quasi-free nucleon-nucleon collisions. New
results on the spin observables for the vec(p)p --> pp eta reaction, combined
with the previously determined total cross section isospin dependence, reveal a
statistically significant indication that the excitation of the nucleon to the
S11(1535) resonance, the process which intermediates the production of the eta
meson in the nucleon-nucleon interactions, is predominantly due to the exchange
of the pi meson between the colliding nucleons.
