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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).
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