  Within the framework of the so-called "valon" model, we argue that a
substantial part of the nucleon spin, about 40%, is carried by the polarized
valence quarks. The remaining is the result of cancellations between gluon
polarization and the orbital angular momentum, where the gluon polarization is
the dominant one. It is shown that the sea quark contribution to the spin of
any hadron is simply marginal and consistent with zero. Our findings point to a
substantially smaller value for a_8 thaninferred from hyperon- $\beta$ decay,
suggesting that fullSU(3) symmetric assumption needs to be reconsidered. New
and emerging experimental data tend to support this finding. Finally, we show
that within the model described here the experimental data on the polarized
structure functions $g_{1}^{p,n,d}$ are reproduced.
