Illustration of the typical evolution of a Mestel disc in one N–body simulation, via the surface density (up to R​ = ​8R₀) at two different times. During the collisional phase (left), the disc remains reasonably axisymmetric, displaying recurrent weak transient spirals. After the phase transition (right), the disc develops strong bisymmetric fluctuations, which ultimately saturate (Figure 1), up to the late formation of a bar (see Figure 10 in F+15).