Fig. 7

Transmission electron microscopy images of P. californica juveniles 20 to 25 min after completion of inversion. Heavy metal-stained sections of fixed P. californica juveniles 20 to 25 min after completion of inversion (still inside the mother spheroid). a Transverse section; cells with trapezoid contours; CBs are located at the basal halves of the cells; the beginning of ECM biosynthesis is already visible at the outer edge of some cells. b Higher magnification of boxed area in a; inset: even higher magnification of boxed area in the main image showing a single CB. Black arrowheads: embryonic vesicle; yellow arrowheads: CBs; magenta arrowheads: flagella; ECM: extracellular matrix; N: nucleus. Scale bars: (a, b) 5 μm; (inset in b) 2 μm