 (C) Still frames from time-lapse recordings of an embryo expressing a myosin-II probe (Sqh::GFP). Embryos were mounted vertically to image the transverse cross-section using two-photon microscopy. At the onset of gastrulation, myosin-II localized to ring structures representing the leading edge of the cellularization front (lower arrow). During ventral furrow formation (torques open rectangle), myosin-II accumulated at the apical side (upper arrow) of the cells that invaginate and the basal pool was progressively depleted. Scale bar, 40 µm 