Fig. 4. Outer cells migrate dorsolaterally as a group. (A) Overview of a time-lapse sequence of a Drosophila embryo showing the initial epithelial tube (0:15:00), the EMT and first round of division (0:21:00), a migratory phase (0:31:00) and the onset of the second division (0:51:00). Lower panels give reconstructed cross-sectional views. (B) Following the first division, there are typically three to four rows of outer cells (i.e. cells that are in contact with the ectoderm) on either side of the midline, which move laterally over the ectoderm. Two cells on either side of the midline (white dots) are tracked for a period of 12 minutes, as they move apart (see Movie 2 in the supplementary material). This image (and C and E) show a single focal plane. Cells marked with a black dot have, by the last panel, moved dorsolaterally into a deeper focal plane (data not shown). (C) A more superficial focal plane showing the movement of outer cells in regions closer to the midline. The positions of the cells marked in B (white dots) (not visible in this focal plane) are here again depicted with white dots. The original location of these cells with respect to the ectoderm was tracked and is indicated with a black dot. As the marked cells move laterally, other cells (down arrowheads) move laterally to occupy the original positions of the marked cells on the ectoderm. Times are as in B. (D) A reconstructed cross-section showing the uneven nature of the ectodermal surface over which the cells migrate laterally. Rounded regions appear to be nascent neuroblasts (asterisks). The indicated cell (0:43:00, arrowhead) corresponds to the cell in C (arrowheads on right-hand side). Times are as in B. (E) Enlarged view of the sequence in B showing that the marked cell (white dot) maintains its relationship to neighbouring cells (arrowheads) during migration. The cell to the lower right of the marked cell moves into a deeper focal plane (data not shown). Scale bars: 20 µm.