During development, the cells of an embryo divide until a fully functional organism emerges. One component of the cell is especially important during this process: the cell cortex. This fine network of hair-like filament structures (called actin) just below the cell membrane is the main determinant of cell shape and is involved in almost everything a cell does, such as moving, dividing, or sensing its environment. Yet, the cortex must first be built from single molecules, and if it is not built just right, the cells of an organism would never get to the right place to perform their functions.