Two-stage Model. A. Filament growing in medium with nitrogen source is postulated to be in nonhomogeneous state, some cells predisposed to differentiate by reason of their positions in the cell cycle (represented by clock time). After fixed nitrogen is removed from the medium, some vegetative cells (green) differentiate into preheterocysts (yellow). Some of these continue on to full maturity (yellow with white border) while others regress back to vegetative status.