  We analyze propagation equations for the polar modes of gravitational waves
in cosmological space-times. We prove that polar gravitational waves must
perturb the density and non-azimuthal components of the velocity of material
medium of the Friedman-Lemaitre-Robertson-Walker spacetimes. Axial
gravitational waves can influence only the azimuthal velocity, leading to local
cosmological rotation. The whole gravitational dynamics reduces to the single
"master equation" that has the same form for polar and axial modes. That allows
us to conclude that the status of the Huygens principle is the same for axial
and polar gravitational waves. In particular, this principle is valid exactly
in radiation spacetimes with the vanishing cosmological constant, and it is
broken otherwise.
