  In this paper, we study the role of shear fields on the evolution of density
perturbations embedded in a Friedmann flat background universe, by studying the
evolution of a homogeneous ellipsoid model. In this context, we show that while
the effect of the shear is that of increasing the growth rate of the density
contrast of a mass element, the angular momentum acquired by the ellipsoid has
the right magnitude to counterbalance the shear. Finally, our result show that
initial asphericities and tidal interaction induce a slowing down of the
collapse after the system has broken away from the general expansion, in
perfect agreement with the previrialization conjecture (Peebles & Groth 1976;
Davis & Peebles 1977).
