Ventral furrow formation was inhibited upon photo-stabilization of myosin-II levels at the cell base. Embryos co-expressing the optogenetic module and the myosin-II probe Sqh::mCh were mounted with the ventral tissue facing the objective. A region of photo-activation (two-photon λ=950 nm) was defined at the cell base in the anterior half of the embryo, and whole-cell myosin-II signal was recorded (confocal setup; λ=561 nm) in an alternating fashion. Scale bars, 20 µm (C, D) z-projections (transverse cross section) of myosin-II time-lapse recordings of the non-activated (C) and activated region (D). While cells in the non-activated region internalized, resulting in a complete closure of the furrow (C), stabilization of basal myosin-II levels prevented invagination (D). Scale bars, 20 µm 