(A) Vertex resolution during axis elongation in an embryo expressing Resille:GFP. Blue indicates the new DV interface, red labels the anterior and posterior cells. (A’) Kymograph illustrating the elongation of the DV interface shown in (A). Scale bar, 10 s. The interface is rotated by 90° with respect to (A). Anterior down, dorsal left. (B) Rates of change for edge length (blue, solid line), anterior cell area (red, dashed line), and posterior cell area (red, dotted line) during the neighbour exchange event shown in (A). Rate of change was calculated with respect to t + 60 s. (C) Correlation coefficients between changes in edge length and changes in anterior or posterior cell area (n = 220 pairs in 110 neighbour exchange events in 13 embryos). (D) Changes in correlation between edge length and anterior (dashed) or posterior (dotted) cell area during the neighbour exchange event shown in (A) when the edge length signal was shifted in time in 10-s increments. Arrowheads indicate the correlation minima (blue) or maxima (red) closest to 0-s shift. (E) Distribution of time shifts (absolute value) required to obtain the minimum (blue) and maximum (red) correlations in all 220 signal pairs shown in (C). (F, G) Rate of change in cell area in embryos injected with water (F, n = 122 cells in 3 embryos) or 100 mM Y-27632 (G, n = 99 cells in 3 embryos). Each line represents a single cell. (H) Oscillation amplitude for changes in cell area in embryos injected with water (blue) or 100 mM Y-27632 (red). Asterisks indicate p < 0.001. (I–J') Vertex resolution during axis elongation in embryos expressing E-cadherin:GFP and injected with water (I) or with 100 mM Y-27632 (J, J’). Arrowheads indicate nascent DV interfaces. (K) Distribution of vertex resolution angles relative to the AP axis in embryos injected with water (blue, n = 28 vertices in 3 embryos) or 100 mM Y-27632 (red, n = 25 interfaces in 3 embryos). Angles were measured 150 s after the onset of vertex resolution. An angle of 90° with respect to the AP axis corresponds to the DV axis. (L) Length of new DV interfaces forming within 30° of the AP axis in embryos injected with water (blue, n = 25 interfaces in 3 embryos) or 100 mM Y-27632 (red, n = 11 interfaces in 3 embryos). (A, I–J’) Anterior left, dorsal up. Scale bars, 5 µm. (B, F, G, L) Time is with respect to the onset of vertex resolution, defined as the first time point in which the length of the nascent interface exceeded 1 µm. (H, K, L) Error bars, s.e.m. AP, anterior-posterior; DV, dorsal-ventral.