Details of embryos 10 min to 35 min after the initiation of inversion (mid-inversion stage). SEM images of isolated embryos and LM and TEM images of physical midsagittal cross-sections (see Figures 3D, E, F and 4D, E, F for an overview). (A) SEM view of the bend region; two broken lines encompass the region with maximum curvature; a rectangle indicates an image detail shown in the inset; and arrowheads point to some of the numerous CBs. (B) SEM view of the opening of the phialopore; the anterior hemisphere is in the upper right corner, and the already inverted posterior hemisphere is in the lower left corner (highlighted in green); passively stretched cells bordering the phialopore are colored in blue; and CBs are colored in red. (C, D) LM images of physical midsagittal cross-sections of the bend region. The developmental stage in (D) is approximately 10 min later than the stage in (C); during this period, the radius of the curvature in the bend region increases significantly as indicated; and a rectangle indicates an image detail shown in (F). (E) LM image of a physical midsagittal cross-section with cells of the anterior hemisphere; the inside and outside of the embryo are indicated. (F) TEM image of a physical midsagittal cross-section of the bend region showing the position of CBs (arrowheads) (see D for an overview). Scale bars: (A, E) 3 μm; (B, C, D) 5 μm; (F and inset in A) 1 μm. CB: cytoplasmic bridge; LM: light microscopy; SEM: scanning electron microscopy; TEM: transmission electron microscopy.