that only the elements produced by type II SNe (mostly $\alpha$-elements and some iron) can escape the star-forming regions. We have made this choice following the conclusions of Marconi et al. (1994) and Kunth et al. (1995), who showed that models with differential wind can explain better the observational constraints of blue compact galaxies in general and IZw18 in particular. The justification for the existence of differential galactic winds can be found in the fact that during short starbursts type II SNe dominate. Since SNII explode in association, they are likely to produce chimneys which will eject metal enriched material (De Young and Gallagher, 1990). On the other hand, type Ia SNe are not likely to trigger a wind since they explode mostly during the interburst phase and have a large range of explosion times (from $3 \cdot 10^{7}$ to a Hubble time) inducing them to explode in isolation.

The terms on the right side of equation (1) represent, respectively, the rate at which the gas is lost via astration and galactic wind and the rates at which the matter is restored to the interstellar medium (ISM) by:

\(a) single stars with masses between $M_L=1.0M_{\odot}$, which is the lowest mass 