Abstract: An apparatus in which flowing developer material is mixed. A plurality of spaced vanes project outwardly from a dividing plate to define a plurality of sets of chambers arranged in substantially parallel rows. Each chamber is arranged to receive developer material at an entrance aperture and to release the developer material from an exit aperture with the exit aperture being displaced from a position vertically below its entrance aperture. Each row has an alternate series of chambers with their exit apertures spaced respectively to the left and right of the position vertically below their entrance apertures. The entrance apertures and exit apertures of the chambers of different rows are aligned so that on successive recirculations of the developer material, the developer material moves simultaneously from left to right and from row to row, and from right to left and from row to row.