A contractile surface model predicts shape changes during the SCW.

A: For each time step a surface Hamiltonian for a spherical shape with varying surface tension is minimised. The surface tension follows a Gaussian profile that is moved over the surface with changing strength. The predictions of the model in B show similar features as a zoom-in to the experimental curvatures during the SCW in C. D: The contractile model can also predict tangential flows in the cortex, that in turn can couple to the hydrodynamic flow, in addition to flows caused by the radial deformations as explained in Fig 4. The tangential surface movement due to the Gaussian-shaped local contraction of a viscous medium is analytically calculated. The tangential surface movement is used as an input for the hydrodynamic model. The effect of different contraction positions on the flow pattern is small. This can be seen from the flows (arrows) due to a rotationally symmetric surface contraction (red colours) around the AV-axis (star).