Such a spatially homogeneous oscillating system can convert into a periodic pattern that is stable in time if, at any location, an A- P border has been formed. Imagine that all cells are in the P state except of the most anterior cells that are in the A state. Cells close to this border stabilize each other while the P cells distant to this border switch from P to A, forming in this way a second A/ P border, and so on. As shown in the simulation above, the periodic pattern can be elongated by terminal growth.
But what gives rise to the first border? To account for several observations, it is assumed that some sort of positional information provides the driving force for this oscillation. Imagine a field of cells exposed to a graded morphogen concentration, which has its high point at the posterior end, and further, that all cells are in the P state. Cells above a certain threshold concentration switch from P to A, a process that leads to the first P- A border. Again, cells distant to the new P/ A border will switch back to A, forming the next A/ P border, and so on. After each full cycle, one pair of A/P stripes is added. In the course of time, the region of the stable spatially alternating A- P pattern enlarges at the expense of the cells that still oscillate between A and P. The borderline between the stable and oscillating cells move over the field in a wave-like manner. Since the oscillation is driven by a gradient with a high point at the posterior end, the transitions occurs a bit earlier there and sweeps towards anterior in a wave-like manner. It comes to rest at a certain distance from the last-formed border. Thus, although the somites separate from each other in an anterior to posterior sequence, it was predicted that this is based on
oscillations with a wave-like phase shift causing an apparent wave in posterior-to anterior direction. The dynamics of the c-hairy1
expression pattern as observed by Palmeirim et al.  in chickens corresponds the expected pattern for the P-state.