Details of embryos 5 min to 10 min before the initiation of inversion (pre-inversion stage). cLSM, SEM and LM images of intact or fragmented embryos, and an LM image of a physical cross-section. (A) LM side view of a juvenile inside the intact parental spheroid. Arrowheads point to minor dents that quickly appear and disappear immediately before the initiation of inversion. (B) Midsagittal cLSM optical cross-section of an embryo showing the localization of F-actin (red) using phalloidin-TRITC (see Figure 5A1 for an overview). (C) Midsagittal cLSM optical cross-section of an embryo showing the localization of both F-actin (red) and nuclei (blue) with a phalloidin-TRITC/DAPI overlay (see Figure 5A3 for an overview). (D) LM image of a physical midsagittal cross-section. Arrows indicate the viewing direction in (E) and (F) (see Figure 4A for an overview). (E) SEM view of the cell monolayer observed from outside an embryo (see D and Figure 3B for an overview); arrowheads point to some of the numerous CBs. (F) SEM view of the cell monolayer observed from inside a fragmented embryo (see D for an overview). Scale bars: (A) 20 μm; (B, C, D) 10 μm; (E, F) 3 μm. CB: cytoplasmic bridge; cLSM: confocal laser scanning microscopy; DAPI: 4',6-diamidino-2-phenylindole dihydrochloride; F-actin: filamentous actin; LM: light microscopy; SEM: scanning electron microscopy; TRITC: tetramethylrhodamine B isothiocyanate.