Abstract
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, so as 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 during the stage of the evolution of the Universe when structures were formed. The most logical conclusion is that if magnetic fields existed in those times then their magnitudes were too small to modify the Jeans mass. Our results place limits on the possible values of seed magnetic fields consistent with the formation of structures in the Universe. These values are within the range of the results obtained by other authors.
