Surface density Σ(r, θ) of the gas disc with the azimuthally averaged profile Σ(r) subtractedto clearly see the respective spiral arms, the corotation region, and other perturbations in the surroundings of the Hill sphere. The system was evolved for 100 orbital periods P_orb (at 5.2 au) so that the hot-trail effect can develop and increase the eccentricity. The situation is very different for a single 0.1 M_⊕ embryo (top panel), with a very regular spiral, and for 120 embryos with the same masses (bottom panel), with spiral arms overlapping each other and creating an irregular overall pattern. The situation corresponds to Fig. 2, but it is much easier to see the perturbations when Σ − Σ (r) quantity is used. The resolution 3072 × 4096 was used for the former short-term simulation, and 2048 × 3072 for the latter. The Hill spheres are shown as small black circles.