A round baler is disclosed for forming crop material into cylindrical bales. The baler has a main frame (11), a pair of side walls (19), a crop pick-up (16) mounted on the main frame (11), and a tailgate (14) pivotally connected to the main frame (11). The tailgate (14) is operative between a closed position during which a bale is being formed in an expandable chamber, and an open position during which a formed bale is being discharged from the chamber. A sledge assembly (20), pivotally mounted on the main frame (11) for movement between a bale starting position and a full bale position, has crop engaging transverse rollers (21-23) for urging the crop material along a spiral path in the chamber for starting and forming a bale. An apron (32) is supported along a continuous path on the main frame (11) and tailgate (14) by a plurality of rotatable guide members (31, 33-36, 43-44). The path has an inner course (c) that cooperates with the rollers (21-23) on the sledge assembly (20) to define moveable walls of the chamber. A drive roll (38) moves the apron (32) along the continuous path. A take-up assembly, affixed to the sledge assembly (20), moves therewith between the bale starting position and full bale position to provide slack in the apron (32) under conditions where the tailgate (14) is moved from its closed position to its open position, thereby causing the drive roll (38) to become disengaged from the apron (32).