each longitudinal side of the first conveyor length 28 a retaining belt 43 the retaining assembly 57 engages the length 28 so that the lengths 28 and 29 provide compartments 58 relatively fixed in configuration the retaining belts 43 have their transverse width extending generally upwardly and pass between pairs of rollers 44 of which at least one is driven more particularly the belts 43 engage the belt 26 so as to maintain the configuration of the belt 26 over the first length 28 once the belt 23 has been deformed to accommodate a bag 14 accordingly the bags 14 are each confined in a generally rigid compartment 58 each compartment is defined between opposing conveyor surfaces 59 and 60 in the embodiment of fig1 to 7 the plate 42 and belt 26 are longitudinally divided so that the belt 26 has two coextensive endless belt members 46 providing the first belt length 28 with the belt members 46 spaced by a gap 47 the plate 42 has a longitudinal slot 58 adjacent the gap 47 in this embodiment there is provided in combination with the conveyor assembly 18 a detection device 44 to detect bags that leak the device 44 includes a detector 45 as seen in fig6 the detector 45 includes an arm 48 that projects through the gap 47 and slot 58 to engage the bag 14 the detector 45 generates a signal indicative of the.