Surface density of the discs in runs 1, 2, and 3, at 22 kyr (see Table 2). Panel a: disc has z = 1 (solar metallicity), and is shown just prior to the formation of a bound fragment. Panel b: disc has a metallicity reduced by an order of magnitude (z = 0.1). The disc is unstable but the spiral arms are not as strong as in disc in run 1. Panel c: disc has a metallicity increased by an order of magnitude (z = 10). No strong spirals have formed yet. Spirals do eventually form, but the disc does not fragment due to a period of rapid expansion.