  The behavior of perturbations is studied in cosmological models which consist
of two different homogeneous regions connected in a spherical shell boundary.
The junction conditions for the metric perturbations and the displacements of
the shell boundary are analyzed and the surface densities of the perturbed
energy and momentum in the shell are derived, using Mukohyama's gauge-invariant
formalism and the Israel discontinuity condition. In both homogeneous regions
the perturbations of scalar, vector and tensor types are expanded using the
3-dimensional harmonic functions, but the model coupling among them is caused
in the shell by the inhomogeneity. By treating the perturbations with odd and
even parities separately, it is found, however, that we can have consistent
displacements and surface densities for given metric parturbations
