  The phenomenon of scaling in deep inelastic lepton-nucleon scattering is
usually explained in terms of the Feynman parton model, and the logarithmic
corrections to scaling are explained in the framework of perturbative QCD. For
testing the validity of the parton model, we consider the deep inelastic
electron scattering in a model in which the system electromagnetic current
operator explicitly satisfies relativistic invariance and current conservation.
Let the struck particle have the fraction $\xi$ of the total momentum in the
infinite momentum frame. Then it is shown that, due to binding of particles in
the system under consideration, the Bjorken variable $x$ no longer can be
interpreted as $\xi$, even in the Bjorken limit and in zero order of the
perturbation theory. We argue that, as a result, the data on deep inelastic
scattering alone do not make it possible to determine the $\xi$ distribution of
quarks in the nucleon.
