Abstract: A container assembly (10) comprises a vessel (21) for containing a biological medium, gametes and/or one or more embryo(s). The vessel (21) has a CO2 permeable seal (28) and a closure device (30) for selective access. A buffer chamber (60) for a CO2 enriched atmosphere at least partially surrounds the vessel and is defined by a shell (61). The buffer chamber is in communication with the CO2 permeable wall. Such a container assembly is particularly adapted for IVC in which case the permeable seal (28) prevents ingress of vaginal secretions. The buffer chamber (60) mediates the aqueous pH in the vessel after the container assembly is removed from a CO2 enriched environment.