Internalization events organize local and global tissue movements. (A) Three-dimensional (3D) projection stills showing movement of ABplpap while covering ABalppp. White shade marks ABplpap, while white circle and plus (+) sign mark ABalppp and its apical contractile flows during internalization. Black star marks the rosette covering Ea/Ep, in which ABplpap participated initially but left. (B) Schematic representation of lineage groups before and after the establishment of superficial left/right symmetry. Dashed lines represent the midline. (C) Movement paths of bilateral AB cells (ABalp, ABara, ABplp, ABprp) at AB64 and AB128). Th path of each cell was averaged over five lineaged embryos, and the thin end marks the start position. The paths of the sister cells have beencombinedinto a single path for visual clarity. Dashed lines represent the midline, which moved toward the anterior-posterior (A-P; long) axis of the embryo as cells moved. Dashed circles mark the remaining gastrulation cleft. (D) Individual migration paths for the ABprppp cell from five lineaged embryos. (E) Correlation analysis of movements; the matrix heat map representation shows correlation of paths of individual AB cells (same cells as in (C)) at the AB128 stage along the left/right axis. (F) Bilateral AB cells moved towards the midline as they covered MS cells. (Left) Three-dimensional (3D) projection image at AB64. The dashed line marks the leading edge of the AB cells, and arrows show direction of movement. MS cells can be recognized by their apical contractile flows. (Right) Schematic representation of continued AB movement in the next cell cycle to finish covering the MS cells. (G) Anterior movement of AB cells on the right side of the embryo (ABprp) upon two internalizations in front of the movement. White shade marks three AB cells, and white circles mark internalizing cells. The black line in the final frame marks the complete sealing over of an internalizing cell, while star marks the other internalization.