Fig. 9. Model for mesodermal spreading in Drosophila embryos. (A) The invaginated epithelial tube before the EMT. (B) The mesodermal cells spread down onto the ectoderm as the EMT and first division occur. (C) Outer cells polarise and proceed to migrate dorsolaterally as a group (blue). As they move away, other cells take up their positions on the ectoderm (white arrows). Inner lateral cells (green) are attracted to the dorsal ectoderm and move over the outer cells. (D) During the second mitosis, inner medial cells (orange) that have failed to contact the mesoderm intercalate into the outer cell layer. (E) The monolayer is formed. See Discussion for details.