An instant film unit, including first and second superposed sheets, and conventional internal spacers for controlling the depth of a layer of processing fluid distributed by flowing the fluid between the sheets, is provided with additional internal spacers that reduce the resistance to fluid flow and augment the quantity of fluid supplied to the lateral edges of the imaging area. In the disclosure, the first spacers comprise portions of a mask, which frames the imaging area. The additional spacers are side rails which separate the lateral edges of the sheets by an amount greater than the thickness of the mask. The film unit is adapted for use with a processing device which includes a pressure nip long enough to span the imaging area, but the side rails are outside of the pressure nip adjacent the ends thereof.