  We discuss the relationship of the forward matrix element of the operator
$\bar\psi\psi$, related to the so-called sigma term, to the quark number. We
show that in the naive quark model in the canonical formalism these quantities
coincide in the limit of small average quark momenta. In the QCD parton model
defined through light-front quantization this result is preserved at leading
perturbative order but it receives radiative corrections. We analyze the
theoretical and phenomenological consequences of this result, which provides a
bridge between a current algebra quantity, the sigma term, and a deep-inelastic
quantity, the parton number.
