The fate of adventitious cartilage on the membrane bones of the skull of the common fowl has been studied from hatching to 4.5 months after hatching. Adventitious cartilage is not a permanent feature of the membrane bone on which it develops but is ultimately replaced, partly by bone and partly by the thickening and chondrification of the fibrous articular membrane. This fibrocartilage, formed in the fibrous articular membrane, forms the adult articular cartilage. Several new areas of adventitious cartilage arise in the fowl after hatching and these have been described. The articular cartilage of the pterygoid-parasphenoid articulation is not replaced by a fibrocartilage but is transformed into a fibrocartilage in the hatched fowl.