All bony fish have a swim bladder, at least at one stage of their development. In teleosts, Dipnoi, and Polypteridae, the swim bladder and the lungs develop as outgrowths from the wall of the foregut, with the primodial air-sac/lung becoming evident very early. Molecular oxygen constitutes the greatest proportion of the gas mixture secreted into the swim bladder of fish, with nitrogen, carbon dioxide, and rare gases such as argon constituting relatively smaller proportions. Swim bladders are lacking in many groups of fishes such as sharks and rays. Where they occur, swim bladders perform multiple functions. A multiplicative process in the countercurrent system of the rete mirabile is thought to explain the secretion of gases into the swim bladder. In some swim bladders, the gas-gland cells form a single layer of columnar cells. On the juxtaluminal aspect, the cells join across conspicuous tight-junctions and much deeper across desmosomes.