Flask has 3 baffles on the lower outside edge of the flask to maximize oxygen transfer to the culture medium when used on an orbital shaker. The polypropylene vented caps have a 0.22µm (PTFE) hydrophobic membrane to allow sterile gas exchange while preventing contamination. Since these caps are vented, the flasks can be autoclaved with the cap attached.Please Note: Glass Shake and Fernbach flasks are not designed for use with a SOLID screw cap for culture of anaerobic microorganisms. (Pressure resistant bottles, flasks or tubes are recommended).