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Fig. 4 Lateral ectoderm and animal ectoderm undergo differential thinning (A–D) Ectoderm thickness as a function of developmental time at the animal pole (B) or the lateral side (C) of the gastrula. In (A), developmental time in minutes (min) is shown as a fire colormap, and relative position of the LME along the animal-to-vegetal extent of the ectoderm is shown as a green gradient representing the percentage of total time points. In (B) and (C), ectoderm thickness is shown at the animal pole and in 100-μm-wide bins lateral to the animal pole (x axis) (B) and in the most marginal ectoderm (100–150 μm from the embryo equator) and 100-μm-wide bins animal to the margin (x axis) (C). Values are shown as mean (solid line) with SD (light-gray area). Green gradient bar represents the presence of LME in a specific position (see also A). Number of embryos: animal pole view (B), 6; lateral view (C), 6. In (D), for each embryo, bins were defined as either lateral ectoderm (Lat Ecto, blue), in cases when the ectoderm was in contact with the LME at least for one time point during the imaging period, or animal ectoderm (An Ecto, yellow), in cases when the ectoderm was not in contact with LME during the entire imaging period. Thickness is shown as ratio to the thickness at the onset of LME migration (t0). Values are shown as mean (solid line) with SD (light-gray area). Number of embryos, 12. Statistical test, two-way ANOVA: ∗∗∗∗p < 0.0001. See also Video S6. (E–G) Model of the ectoderm thickness as a function of developmental time. Ectoderm tissue is modeled as a passive fluid with patterned viscosity from the animal pole (lowest) to the margin (highest) and subjected to an external constant pulling force (fA), which causes an increase of tissue length with a speed of ∼1.5 μm/min (corresponding to the experimentally measured blastoderm epiboly speed) (E). In (F), animal (An Ecto, yellow) and lateral (Lat Ecto, blue) ectoderm thickness is shown as ratio to the thickness at t0. Animal ectoderm corresponds to 200 μm at the animal pole, while lateral ectoderm corresponds to 200 μm at the margin. In (G), ectoderm thickness was quantified as a function of time (90 min, fire colormap) at the animal pole and 400 μm lateral to it (x axis)