  An important ingredient of parton or string cascade models for
ultrarelativistic heavy-ion reactions is a parton description of the baryon.
Whereas previous models needed the concept of a diquark in an essential way, we
have developed a new model using Dirac's approach of Poincare-covariant
many-body dynamics with constraints.
  In our model, the baryon is described as a dynamical set of three valence
quarks and a fourth 'particle', the "junction", which carries the momentum
fraction of the sea quarks as well as all of the glue.
  The model's parameters are the quark current masses, and one interaction
strength, determined by the proton radius. Thus the model has no adjustable
free parameters. Nevertheless, we obtain a remarkably good fit to the valence
quark structure functions of the baryon
