(A) Nuclear staining of M. abdita embryos prior to dorsal closure reveals three types of tissues: the extraembryonic serosa (magenta), the extraembryonic amnion (blue), and the embryonic epidermis (gray). (A’) An orthogonal re-sliced stack along the dashed yellow line in A shows the position of the embryonic (gray), amniotic (blue), and serosal (magenta) tissues in transverse view. (B) Schematics depicting the organization of the serosa cells (magenta), amnion cells (blue), and embryonic epidermis (green) in lateral and (B’) transverse view. The black dashed line represents the vitelline envelope (C) M. abdita embryo undergoing rupture and retraction of serosal tissue along the ventral side. Staining against β-tubulin in green, and DAPI nuclear counterstain in magenta. (D) Serosal cells (magenta arrowheads) accumulate on the dorsal side of the embryo after rupture. The serosa remains apposed to the amnion (blue arrowheads), which is in turn apposed to the embryonic epidermis (white arrowheads). Phalloidin stain in green and DAPI nuclear counterstain in magenta. (E) Images from a time-lapse sequence of serosa retraction in a M. abdita embryo injected with the fluorescent lipophilic dye FM 4–64 (from Video 1). Yellow dashed line shows the contour of the serosa covering the embryo during retraction. (F) Relative changes in area of the serosa during retraction (red, n = 15 embryos), and relative changes in height (h) of the dorsal opening (blue, n = 15 embryos) during dorsal closure in M. abdita. Vertical bars represent standard deviation (SD). Time range of dorsal ridge fusion is represented by pink area (n = 15 embryos) as a landmark for the initiation of dorsal closure. The origin of the time axis (T = 0 min) is set at the point of serosa rupture. (G) Schematics depict transverse views of embryos during serosa rupture, retraction, and dorsal accumulation. Serosal cells rupture along the ventral end of the embryo (red arrowhead). (G’) The remaining lateral serosa cells retract towards the dorsal side (black arrows). (G’’) Serosa cells continue retracting until they completely accumulate onto the dorsal opening (black arrows). Color scheme as in B. In all embryos and schematics, dorsal is to the top. Embryos in A, C, D and E show lateral views where anterior is to the left.