FIG. 4.

Model for tier formation in M. xanthus. The panels represent a sequence of events visualized from left to right. The top portion of each panel displays a top-down view of aggregates, while the bottom portion of the panel shows a side view of the aggregate. The blocks in the side view represent cell swarms traveling along the agar surface. In the first panel, colliding swarms moving in opposite directions intersect and cause one of the swarms to orient at a slight vertical pitch to cross over the other swarm (left arrow), generating an inclined plane. As the swarms continue to converge, cells stream along this vertical alignment and then realign horizontally with the cells below them, thereby forming the first tier (second panel from left). More cells stream into the first tier, increasing its surface area. In addition, some cells continue the vertical ascension through the first tier to initiate the second tier (third panel). This process can continue to form multiple tiers (fourth panel).