Oscillations in medial and junctional myosin levels drive progressively stronger contractions in NBs. (A) Cycles of medial myos­in assembly and disassembly during early and late NB ingression. The cell membrane is labeled in red (GAP43::mCherry) and myosin in green (Sqh::GFP). Dashed circles track a pulse of medial myosin II assembly and disassembly during early and late ingression. (B and C) Representative plots of medial myosin levels versus apical area (B) and rates of medial myosin and area change (C) during ingression. (D and E) Myosin data are shifted backward and forward in time to calculate cross-correlations between changes in medial or junctional myosin and cell area during ingression. R₀ is the correlation coefficient with no time shift. A typical result is shown in E. (F and G) Maximum cross-correlation coefficients (F) and time shift distributions (G) as indicated in D and E. n = 63 NBs, 15 embryos. IQRs for medial myosin were −0.51, −0.45, and −0.34 and for junctional myosin were −0.47, −0.38, and −0.27. *, P = 0.01 (two-tailed t test). (H and I) Mean medial and junctional levels of myosin in an NB (H) and a temporally matched NIC (I). (J) Mean medial and junctional myosin levels during early ingression (up to last 10 min), late ingression (last 10 min), and in temporally matched NICs. n = 22 cells per condition. ***, P = 0.001; ****, P = 2 × 10−⁹; ns (not significant), P = 0.5 (left) and 0.7 (right; two-tailed t test). Data presented are means ± SD. a.u., arbitrary units. (K and L) Expansion of the apical cortex after laser ablation in NBs and NICs in endo-DEcad::GFP embryos. Maximum relative apical expansion (vertical dashed line in L): 97.0 ± 8.6% for NBs (n = 39) and 41.6 ± 3.3% for NICs (n = 36). P = 2.28 × 10⁻⁷ (two-tailed t test). Data presented are means ± SEM. (A and K) Bars, 5 µm.