Fig. 2. Gradual development of coordinated sequences. Example data from continuous recording of movements in a single embryo 16-19 hours AEL, using muscle imaging. Before 17 hours AEL, muscle contractions occur as isolated twitches and unilateral rapidly propagated sequences across segments (arrowheads), which in intact embryos cause rolling within the vitelline membrane. At 17 hours AEL, there is a burst of activity (brief repeated muscle contractions occurring asynchronously in all abdominal segments, with little side-to-side coordination). Later bursts of activity contain motifs resembling elements of forward or backward crawling, in that muscles on left and right sides contract together, and, in comparison with earlier stages, contractions are relatively prolonged (arrowhead). At 18.25 hours AEL, embryos begin to perform sequences resembling complete waves of larval crawling, together with partial waves typically seen slightly earlier in development (arrowheads).