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Fig. 2 Morphogenesis of posterior tissues. (a), Time evolution of posterior tissue shape from an initial semi-circular shape (gray), as 𝜎C/𝑃𝐶 and 𝜎A/𝑃𝐶 are varied (𝜎A/𝜎P=10, 𝜆𝜇/𝜆𝑄=2). Four time snapshots at 𝑡/𝜏=10,25,50,75 (blue, green, orange and red, respectively) are shown for each parameter combination. The dotted lines qualitatively separate different regimes: isotropic expansion (IE), intermediate regime (IR), and unidirectional elongation (UE). The diffuse violet color indicates the estimated parameter region for wild type zebrafish body axis elongation. (b–c) A sharp increase in capillary stress during unidirectional elongation (arrow in a), leads to an increase of tissue pressure (b) that prevents addition of material from dorsal tissues into the MPZ (b; vanishing Q), halting tissue flows (b; vanishing velocities) and arresting further tissue elongation (c).