Fig. 3

Sequence of in vivo light micrographs and traced contours of an inverting P. californica embryo. In vivo time-lapse sequence of an embryo within its embryonic vesicle (black arrowheads) after mechanical separation from its mother spheroid. a-h lateral view; upper panel (Z1): medial focus plane; middle panel (Z2): peripheral focus plane; lower panel: traced contours of the embryo and its embryonic vesicle. The contours of the respective time point and the preceding time point were superimposed (consequently, there is only a single contour at the first time point). a-h Points in time are given above each column. The start of inversion is at time zero. Red arrows: direction of cell sheet movements. a Before inversion begins the embryo is bowl-shaped and all cells are teardrop-shaped (pink dashed line, also see Fig. 5a). b, c Inversion begins with an outward curling of the peripheral cells; this movement deforms the embryonic vesicle. The apical ends of the peripheral cells become hemispherical, while their basal cell ends become thinner. The cells at the posterior pole of the embryo become spindle-shaped (dashed red line, see also Fig. 5c). d, e br: bend region; asterisk: bent peripheral cell. The posterior pole moves towards the opening of the cell sheet while the peripheral region continues to curl until the plakea is nearly flat. f-h ph: phialopore. The entire cell sheet proceeds to curl so that the previously concave plakea becomes convex. Simultaneously, all cells elongate and adopt an elongated teardrop shape (pink dashed line in h); a transparent chloroplast-free region appears at the apical part of each cell. Scale bars: 20 μm