The present invention is directed to a reversibly closable bag for cultivation of cells and/or tissues, comprising at least one reversibly closable aperture in the bag wall, and a surface-increasing convection means inside said bag, the means being capable to generate and/or modify a convection in a fluid within said bag when at least one of the fluid, the bag, and the convection means is agitated, wherein the surface increasing convection means is selected from at least one of at least one blade, at least one particulate filler, or at least one surface-increasing substrate being made of a single mould. The invention further provides a system comprising at least two bags, wherein the bags are interconnected via at least one aperture in their bag wall, and a cultivation process using such a bag or system, in which at least one type of cells, tissue, tissue-like cell cultures, organs, organ-like cell cultures, or multicellular organisms are cultivated in the presence of at least one fluid or solid medium necessary for growing and/or cultivating the aforesaid culture.
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