The Early Development of the Interorbital Septum and the Fate of the Anterior Orbital Cartilages in Birds

Abstract

ABSTRACT The development of the interorbital septum and anterior orbital cartilages in birds has been studied with special reference to the chick embryo. In the chick the trabeculae are at first separated by a bar of intertrabecular mesenchyme which projects downwards between them to form a prominent keel. Later this mesenchyme fuses with the trabeculae and anterior orbital cartilages (preoptic roots) to form the interorbital septum. In some other Sauropsida (i.e. certain turtles and birds) the intertrabecular bar of mesenchyme is also well developed and has a keel. Although this bar is always partly blended with the trabeculae and orbital cartilages, its appearance is sufficiently distinctive to justify the use of the term intertrabecula for it. In other Sauropsida (i.e. lizards, the sparrow) the trabeculae seem to develop closer together and the intertrabecular mesenchyme is less clearly differentiated. The ventral keel is not well developed, and the use of the term intertrabecula is not indicated. The differences between this and the previous group (chick, turtles, &c.) are, however, probably of degree rather than kind. During embryonic life the olfactory nerves in the chick are temporarily enclosed in cartilaginous tunnels formed mainly by the fusion of the anterior orbital cartilages with the roof of the nasal capsule. The subsequent regression of the orbital cartilages is responsible for the exposure of the nerves in the orbits. The different relationships of the olfactory nerves in certain other birds are shown to depend on relatively slight variations of the structure of the orbitonasal region of the chondrocranium. The formation of the interorbital septum, the growth and regression of the anterior orbital cartilages and the disposition of the olfactory nerves are virtually unaffected by the experimental ablation or partial ablation of one eye at a stage before the onset of skull development. Conditions similar to those in the operated specimens were found in three chick embryos which showed naturally occurring, unilateral microphthalmia.