Fray and fragment scenario of core formation. The three panels represent three different stages in the evolution of a large-scale filament like L1495/B213. In the first stage (left panel), two gas flows collide and produce a filamentary density enhancement. With time (middle panel), a combination of residual turbulent motions and gravity splits the gas into a series of intertwined velocity-coherent fibers. Finally (right panel), some fibers accumulate sufficient mass to reach the limit for gravitational fragmentation and form chains of dense cores.