  It is shown that the use of extended sets of irreducible representations of
the Lorentz group opens new possibilities for the theory of relativistic wave
equations from the point of view of the space-time description of both the
internal structure and the isospin degrees of freedom of elementary particles.
The approach developed in this work also makes it possible to apply the methods
of the theory of relativistic wave equations in superstring and gauge models of
fundamental interactions.
