Fig. 2: Components of the model.

a Particles (circles), bonds (black lines), and forces (red arrows). All pairs of particles, including non-bonded pairs, exert repulsive forces on one another (double-headed arrows, repulsion in two particle pairs, one bonded and one unbonded), representing volume exclusion. Bonded pairs, in addition to their repulsive forces, exert attractive forces on one another (pair of single-headed arrows), representing adhesion. b Particle radius (short arrow) vs. cell radius (long arrow). Dashed circle: target or equilibrium size of the cell. See main text and Methods for details. c Cell shape and cell boundaries are not explicitly represented by the model. But cells are interpreted as epithelial, tiling the layer and leaving no gaps. Therefore, the inferred cell shapes (dashed boundary) are irregular and polygonal, as typically found in epithelia. d Exogenous forces act on the leading edge particles (yellow), pulling them vegetalward (red arrows). In the first version of our model (see Model 1, and Methods), we model a force that in living tissue is presumed uniform along the continuous EVL/yolk boundary. The model itself is discrete, and force acts on individual particles, which are free to be non-uniformly spaced. Therefore, in order to apply constant force per unit length of edge, we weight the actual force applied to each particle proportionally to the average horizontal distance between its center and those of its two bonded leading-edge neighbors. In panel d, arrow thickness represents magnitude of force. e In the later version of our model (see Model 2, and Methods), we include the same weighting shown in d, but in addition, we further weight the applied force according to the vertical position of each leading edge particle (lagging vs. leading). In e, arrow length represents this additional multiplier on the force magnitudes.