  The particle density distribution emerging from the solution of the Vlasov
equation for relativistic, non-interacting particles with spherically symmetric
initial conditions is shown to exhibit a shell-like structure for late fixed
times in the center-of-mass (CM) frame of the system. A similar phenomenon was
recently observed employing the test-particle method to solve the Vlasov
equation for quarks with Nambu-Jona-Lasinio-type interactions, and was
attributed to the attractive forces among the particles. Contrary to this
claim, it is demonstrated here that this effect is of purely relativistic
origin and is sensitive only to the mass of the particles and their initial
phase-space distribution.
