The present invention provides a flask having a top piece 20 and a bottom piece 22 with optical properties on the interior surface 24 of which cells are grown. The top piece 20 and the bottom piece 22 are joined by a removable elastomeric seal 26. By disengaging the elastomeric seal 26 from the flask body 12, the top piece 20 can be separated from the bottom piece 22 providing access to the flask interior.