. "Allocation of epithelial cells within each migrating branch in an open tracheal system to distinct tracheal cell fates. During the migration phase each branch forms a well-defined number of cell types (including fusion cells, terminal cells and branch cells) at precise positions." . . . "epithelial cell type specification, open tracheal system" . "tracheal cell type specification" . "tracheal epithelial cell type specification" . . . . .