  We analyze the effect of pressure on the evolution of perturbations of an
Einstein-de Sitter Universe in the matter dominated epoch assuming an ideal gas
equation of state. For the sake of simplicity the temperature is considered
uniform. The goal of the paper is to examine the validity of the linear
approximation. With this purpose the evolution equations are developed
including quadratic terms in the derivatives of the metric perturbations and
using coordinate conditions that, in the linear case, reduce to the
longitudinal gauge. We obtain the general solution, in the coordinate space, of
the evolution equation for the scalar mode, and, in the case of spherical
symmetry, we express this solution in terms of unidimensional integrals of the
initial conditions: the initial values of the Newtonian potential and its first
time derivative. We find that the contribution of the initial first time
derivative, which has been systematically forgotten, allows to form
inhomogeneities similar to a cluster of galaxies starting with very small
density contrast. Finally, we obtain the first non linear correction to the
linearized solution due to the quadratic terms in the evolution equations. Here
we find that a non null pressure plays a crucial role in constraining the non
linear corrections. It is shown, by means of examples, that reasonable thermal
velocities at the present epoch (non bigger than $10^{-6}$) make the ratio
between the first non linear correction and the linear solution of the order of
$10^{-2}$ for a galaxy cluster inhomogeneity.
