Abstract: An assembly comprises a multiwell test plate having a plurality of wells having an opening and a base surface, an insert plate having a plurality of inserts positioned to align with a corresponding one of the wells whereby the insert plate can be nested with the multiwell plate. Each of the inserts includes a supply port arranged for flow communication with a supply source, an exhaust port, a bottom portion having a plurality of channels extending between the supply port and the exhaust port, and a gas permeable membrane covering the bottom portion. Each of the inserts is sized to position the gas permeable membrane a desired distance from the base surface of the multiwell test plate when the multiwell test plate and the insert plate are coupled to one another.