Simply slide the culture chamber over the glass ring. A hydrophobic semipermeable foil from Dupont that is selectively permeable to gases (O2, CO2), but not to fluid, keeps the culture clean and sterile, preventing contaminations by airborne pathogens. It also greatly reduces evaporation and thus prevents a dry-out of the culture. Autoclavable. Comes complete with membranes. From ALA Scientific Instruments Inc.