In view of the frequent appearance, even in technical literature, of the old-fashioned idea that the lung of the tetrapod has evolved out of the air-bladder of fish, further work on the morphology of the air-bladder seems desirable. The theory of an air-bladder origin of the lung was brought forward by Boas, who based his view on purely theoretical arguments, which have, however, not found support in the facts brought to light by the advance of our knowledge of the embryology of the more archaic vertebrates.