NBs ingress via an anisotropic loss of apical cell–cell junctions. (A) An NB undergoes apical constriction and disassembles its AJs while moving out of the epithelium. (B–D) Stills of a time-lapse video of the ectoderm during stage 8 (membrane marker Spider::GFP). Numbered cells are medial (red, M), intermediate (blue, I), and lateral (yellow, L) NBs, located 1–3, 4, and 7–9 cell rows away from the VM, respectively. White arrowheads indicate the position of NBs that have lost their apical membrane. Insets show the corresponding stage of embryonic axis elongation (black arrowheads, posterior end). Anterior, left; dorsal, up. (E) Stills of a time-lapse video of an NB (apical domain, yellow; ubi-DEcad::GFP). Arrowheads point to new horizontal junctions forming between DV NICs. (F) Watershed segmentation (green outline) of the ingressing NB in E. Bars: (B–D) 15 µm; (E and F) 5 µm. (G) Mean loss of apical cell area during ingression. Time 0 (dashed line) indicates the onset of ingression. (H) Developmental timing of completion of ingression by NB row. Medial NBs complete ingression first, followed by the intermediate and lateral rows. Time 0 corresponds to the onset of cytokinesis in VM cells. Box plot with IQRs and minimum/maximums displayed. *, P = 0.037; ****, P = 0.0001 (two-tailed t test). (I) DV and AP apical cell length during ingression (Spider::GFP, n values as in G). The apical surface shrinks along the DV axis 1.6× faster than along the AP axis. Time 0 is the onset of ingression. Data presented are means ± SEM.