Embryos co-expressing CIBN-PatJ::GFP-pm, RhoGEF2-CRY an the membrane marker GAP43::mCherry (D-I) were mounted with the ventral tissue facing the objective. After cells underwent apical constriction and cell elongation the basal volume of the ventral cells was photo-activated using two-photon illumination and the myosin-II signal was recorded in alternation (D) Cross section (z-projection) showing the plasma membrane and (E) top view showing the apical plasma membrane at the initial time point before photo-activation. At this stage, the central mesodermal tissue has started to internalize and cells were constricted at the apical surface. In an alternating fashion, the base of the cells in the anterior half of the embryo (left) was photo-activated for 100 sec and the GAP43::mCherry signal in the entire field of view was recorded. The furrow further ingressed and closed completely 3 min (F) and 6 min (G) after initial activation, respectively. In the activated region, the tissue failed to internalize and to form a closed furrow both at 3 min (H) and 6 min (I) after initial photo-activation. The width of the central tissue forming the ventral furrow at the initial point of the experiment is 40 µm (D). In the non-activated region 6 min after initial activation the width of the tissue shrank to about 25 µm (G), whereas it remained constant at 40 µm in the activated region (I). Data information: Scale bars, 20 µm 