(*A* and *B*) Representative confocal slices of a 3D zebrafish ectoderm aggregate in planes tangent to the aggregate surface. *A* intersects the surface cells, whereas *B* is at a depth of > 25 μm and intersects a layer in the bulk. (Scale bar, 10 μm.) (*C*) Plot of the projected areas of all cells. Red points correspond to surface cells, whereas black points correspond to bulk cells, and the solid lines represent the mean of the entire dataset. Error bars represent errors on the aggregate mean. (*D*) Comparison of the relationship between *σ*, *γ*, and *β* as obtained from our extended model with *α* = MAX(0,(*γ*/2-*β*)/(2*P*_{hex})), for various surface cell configurations. The dash-dotted line corresponds to compact cell shapes, whereas the dashed and solid lines correspond to surface cells stretched over two or three cells, respectively. For *γ*/*β* < 2, compact cell shapes are optimal and the differential adhesion hypothesis is approximately satisfied, whereas for *γ*/*β* > 2, cells stretched over three interior cells are optimal and cortical tension dominates. (*E*, *Inset*) Magnification of crossover behavior in *D*.

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