These Nucleon treated flasks have an ergonomic closure that requires only a 1/3 turn to open, close, or vent. The two-position vent/close cap has a "Y" mark that allows visual verification of vent position, even when stacked in incubators. Both vent/close and filter caps employ an integral plug design to provide a liquid tight seal with the inside of the flask neck. Filter caps feature a non clogging filter. Slope shoulder design prevents media from sloshing into the neck. The large opening permits access to the entire growth surface with either pipets or cell scrapers. Graduations printed on both sides of flask simplify measurement. Large frosted writing surfaces on both flask shoulders facilitate sample identification. Extra caps in every carton. Nucleon certified for consistent cell growth.