As in Fig. 1, but for massive PSs. Stars of the first, second, third, and fourth generations are represented as red, blue, yellow, and orange circles, respectively. Panels represent: a) formation of FG stars; b) gas accumulation, from a mix of winds of FG massive stars plus pristine gas; c) first core-collapse SN explosions, which trigger the formation of SG stars; d) gas accumulation, from a mix of FG SN ejecta, winds of FG and SG massive stars, and gas remaining from the formation of SG stars; e) formation of TG stars, when SN explosions end; f) gas accumulation, from winds of super-AGB and AGB stars from all generations; g) formation of 4G stars; h) present-day O-Na anticorrelation. This figure is available in color in electronic form.