Fig. 2. Massive star yields of He, C, N and O for different initial metallicities, according to WW95 (filled triangles ) and M92 (open squares ); Solid curves : Z = , dotted curves : Z = 0.1 (for WW95), dashed curves: Z = 0.05 (for M92). There is good general agreement between the two calculations for stars with initial metallicity Z = . He, C and O are primary elements, i.e. their yields are independent of the initial stellar metallicity; the differences between the two sets of M92 yields for stars with M30 are due to the metallicity dependent stellar winds. Nitrogen is produced as a secondary element in both WW95 and M92.