galaxies is presented. A numerical code that simulates the gravitatIonal contraction of a galaxy since the protogalactic phase has been developed. The models are based on a balance between the turbulent energy dissipation of the gas and the energy input from supernovae, the star formation being regulated by the gas conditions through Schmidt's law. The grid includes closed and open models. In the open models the gas accretion is produced from the intergalactic gas or from a surrounding gaseous halo that supplies the gas and absorbs a fraction of the yield, diluting the metallicity of the galaxy. Tests with the observed present- day gas contents and metallicities of the galaxies support the idea of the open models and particularly of the surrounding gaseous halo.