Fig. 6

Transmission electron microscopy images of P. californica embryos during inversion. Heavy metal-stained sections of fixed P. californica embryos during inversion (inside their mother spheroid). a Midsagittal section; cells of an embryo at 3 to 5 min before inversion; apical cell ends face towards the inner surface of the plakea; CBs are located at the equator of the cells or within the apical half of the cells. b Midsagittal section; cells of an embryo at the beginning of inversion; note the pointed basal end of the left cell; CBs are located at the cell equator. c Sagittal section; peripheral cells of an outwards curled plakea at about 8 min after beginning of inversion; note the conical basal end of the cell in the middle (bend region); CBs are located at the basal cell ends. d Sagittal section; elongated cells of an embryo at about 15 min after the beginning of inversion; the flagella face outwards; CBs are located at the basal cell ends. e Sagittal section; basal cell ends of an embryo at about 15 min after the beginning of inversion; inset: rough endoplasmatic reticulum pervades the CBs. f Section of an embryo at about 15 min after the beginning of inversion; the cells are elongated and wedge-shaped; the flagella face outwards; center of the image: transversally sectioned basal cell ends of central cells of the plakea showing numerous CBs; image periphery: longitudinally sectioned peripheral cells of the plakea. g Magnified image of the framed area in (f). Black arrowheads: embryonic vesicle; yellow arrowheads: CBs; magenta arrowheads: flagella; ER: endoplasmatic reticulum, N: nucleus; S: starch grain; Th: thylakoids; V: vesicle-like structure. Scale bars: (a, b, inset in e) 500 nm; (c, d, g) 2 μm; (e) 1 μm; (f) 5 μm