Fig. 3. Overview of mesoderm development in Drosophila. Typical time-lapse sequence of an embryo with three to four segments of photoactivated mesoderm showing cells undergoing two divisions as they spread out into a monolayer (see Movie 1 in the supplementary material). (A) At 15 minutes post-gastrulation, the mesoderm shows a medial seam (arrowhead), indicating that the invaginated mesoderm is still arranged as an epithelial tube. (B) By 33 minutes, the EMT has occurred and the cells undergo a synchronous division, as evidenced by the appearance of mitotic spindle poles (arrowheads) and spindles (arrows). (C) At 1:03, the cells have re-entered interphase and are migrating out over the ectoderm. (D) At 1:19, the second division is in progress (arrows indicate spindles). During this division the mesoderm rapidly extends laterally. (E,F) By 2:15 the monolayer has been achieved and the embryo (E) shows a similar segmentally repeated striped pattern to a control embryo (F) (twist::CD2) (arrowheads). The segmentally repeated variation in brightness is due to the changing thickness of the epidermal/neural tissue, which attenuates the signal from the photoactivated mesoderm (data not shown). (G,H) z-series reconstruction at the end of time-lapse acquisition (G), showing that the mesoderm has adopted the typical monolayer arrangement seen in fixed twist::CD2 control embryos (H). Scale bars: 20 µm.