Adventitious (Secondary) cartilage in the chick, and the development of certain bones and articulation in the chick skull.

The development of a number of articulations in the chick embryo skull, and of adventitious (secondary) cartilages associated with them, is described. The cells of the adventitious cartilages differed from the hyaline cartilage of the chondrocranium in being encapsulated and rapidly becoming hypertrophic. In every case but one the adventitious cartilage was formed in direct association with an articulation. The articulations may have articular cavities (quadrate-quadratojugal; quadratepterygoid; pterygoid-cranium; squamosal-quadrate) or be without these (squamosalotic capsule; pterygoid-palatine; surangular- and angular-Meckel's cartilage). The adventitious cartilage developed in "germinal cells" which, immediately before the onset of chondrification, had been engaged in ossification. Later, the same group of cells often reverted to bone formation, and the adventitious cartilage became partly covered by bone. Where there were articular cavities, fibrous membranes lining the articulations appeared on each side of the cavity and these usually became fibrocartilaginous. These membranes continued into the fibrous layers of the periostea of the elements concerned, while the germinal cells from which the adventitious cartilages were formed became cambial layers continuous with the cambial layers of the periostea. Movement, and mechanical strains resulting from the action of muscles, is obvious at articulations having articular cavities. In those lacking articular cavities, the anatomy of the muscles makes it extremely probable that the site on the membrane bone is pulled upon, or moved against, the cartilage with which the articulation is made. The facts of the development of adventitious cartilage, and of the anatomy of the musculature, are in harmony with the hypothesis that the change in morphogenetic direction of the germinal cells, from osteogenesis to chondrogenesis, is mechanically induced.