A bioreactor for the ex vivo maintenance and growth of mammalian cells includes a disposable, self contained cell cassette, a system manager, an incubator unit matable with a plurality of the cassettes and a processor unit. The cell cassette includes a substantially circular cell growth chamber defined between a substantially planar cell bed and a gas permeable, liquid impervious membrane. A media inlet enters the cell growth chamber communicates at a radially central portion of the cell bed. A plurality of media outlets are radially outwardly spaced from the media inlet by a distance sufficient for cell growth to occur between the inlet and the outlets. The outlets are substantially equiangularly circumferentially spaced about the cell growth chamber.