Fig. 1. Wave propagation in a slug of Dictyostelium discoideum. The slug shown is from a mutant AX2 strain that has been transformed with different variants of GFP expressed under the control of prestalk-specific promotors to enable cell movement analysis (see movie on CD ‘GFP in Motion’ Trends Cell Biol.). (A) Movement of cells inside the slug. GFP-labelled cells were tracked in both the prestalk and in the prespore region over 20 images (400 seconds). The tracks are superimposed on the brightfield image of the slug, the direction of cell movement is indicated by black arrows. The slug is migrating towards the left. (B) Image subtraction of the corresponding brightfield images taken at 100 second intervals reveals the wave front as dark band in the middle of the slug indicated by a white arrow. (C) The same slug 200 seconds later. The wave (white arrow) has moved backwards. Scale bar: 100 µm.