The present invention relates to animal model systems comprising a chimera between an avian embryo and a mammalian organism. The filter is manufactured by providing a pad of non-woven fibrous material, and bonding together adjacent fibres to create a pattern of reinforced fibres in a surface region of the pad. A method for manufacturing a frame structure is also disclosed. The attaching portion includes an enlarged opening which enables air exhausted from an interior room to be directed out between the base and cover members to the exterior atmosphere. The locking member is lockingly received in the lock-receiving portion.