  In this paper we study a simple model consisting of a dilute fully ionized
plasma in the presence of the gravitational and a constant magnetic field to
analyze the propagation of hydromagnetic instabilities. In particular we show
that the so called Jeans instability is in principle affected by the presence
of the magnetic field. A brief discussion is made attempting to assess this
influence in the stage of the evolution of the Universe where structures were
formed. The most logical conclusion is that if magnetic fields existed in those
times their magnitudes were too small to modify Jeans' mass. Our results places
limits of the possible values of seed magnetic fields consistent with the
formation structures in the Universe. These values are within the range of the
results obtained by other authors.
