  We have improved and examined the applicability of the valon model where the
structure of any hadron is determined by the structure of its constituent
quarks. Nucleon structure functions are calculated within this model in the
Next-to-Leading order. The results compare well with the experimental data. The
model handles the bound state problem and the calculations show a flat or
almost flat behavior for $F_{2}$ which sets in at some region of $x\leq
10^{-5}$at fixed $Q^{2}$. The emergence of this behavior is a consequence of
the model and was not put in a priori as a theoretical guess. It seems that
such a flatness can be inferred from HERA data, although, not completely
confirmed yet. A set of parton distributions are given and their evolutions are
tested. Some qualitative implications of the model for the spin structure of
the proton is discussed.
