  The constituent quarks are interpreted as bound states, which have an
internal structure. The quark distributions of the proton are related to those
of the constituent quarks. The experiments support this hypothesis. Likewise
the spin structure of the proton is related to the spin structure of the
constituent quarks. We find that about 30% of the spin of a constituent quark
is given by the valence quark, and 70% are provided by the gluons.
