The Posterior End of Meckel’s Cartilage and related ossifications in Bony Fishes

Abstract

ABSTRACT In modern Dipnoi (Protopterus) the membrane bones are separated by connective tissue from Meckel’s cartilage, and there is no endochondral or perichondral bone. The cartilage grows evenly over its whole extent. In Polypterus a large articular ossifies the posterior end of the cartilage, including the retro-articular process, and spreads into the neighbouring connective tissues. In Elops the joint surface is carried partly by the articular, and partly by the retro-articular, a special ossification of the retro-articular process. In most teleosts (Mugil, Sardina, Trigla) the articular is absent and the angular invades the perichondrium and cartilage to form the joint surface. Special growth zones of flattened cells are formed in the cartilage which by their growth carry the retro-articular, angular, and dentary away from one another, stability of the jaw being maintained by new growth of the membranous parts of the bones. Endochondral bone is reduced or absent in some specialized fishes (Tetrodon, Notopogon). The sesamoid articular of teleosts is a separated part of the angular which gives insertion to the adductor mandibulae muscle. An attempt is made to follow the evolution of Meckel’s cartilage and the related ossifications by a comparison of the early Dipnoi, Crossopterygii, and Amphibia described in the literature with modern forms.