We introduce a subclass of Valk's self-modifying nets. The considered nets appear as statified sums of ordinary nets and they arise as a counterpart to cascade products of automata via the duality between automata and nets based on regions in automata. Nets in this class, called stratified nets, cannot exhibit circular dependences between places: inscription on flow arcs attached to a given place, depend at most on the contents of places in lower layers. Therefore, the synthesis problem has similar degrees of complexity for (ordinary) nets and for stratified nets, hence it is tractable.