a holder for packaging a biological specimen slide is heat formed from a single sheet of plastic , with tray and cover portions foldable to a closed condition along a hinge line . a window opening in the cover portion allows viewing of identification markings on a slide in the closed holder . the window is initially closed by an integrally formed push - out panel which is removed upon use of the holder as a flag of previous usage of the holder to prevent its unintentional reuse .