Cell movements during culmination. Terminal cell differentiation takes place during the culmination stage and is correlated with cellular movements within the organism, as shown here for an early culminant. PstAB cells present in the slug are the first to migrate down the stalk tube and terminally differentiate into stalk cells, hence referred to as primary prestalk cells. They are replaced by pstA cells, which start expressing ecmB and in turn become stalk cells (secondary prestalk cells). PstA cells are in turn replaced by pstO cells. This process continues until all prestalk cells have been incorporated into the stalk.