Fig. 13.

a When a thin shell is bent, the outer part is in a tensile state and the internal part in a compressive state. We assume that cells respond to the strain by growth, which adds a negative surface tension in the cell layer. b Left, a nasal process grown with a constant negative surface tension (Video 19). Right, a nasal process grown with a negative surface tension proportional to the local curvature (Video 21). The process is nonlinear, and it tends to generate elongated features, with flatter sides