In ovoExperiments Concerning the Eye, the Orbit, and Certain Juxta-Orbital Structures, in the Chick Embryo

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Abstract
The influence of the developing eye on the embryonic orbit and on various juxta-orbital structures has been studied in the chick by removing (a) a single primary optic vesicle (17 experiments), (b) both vesicles (52 experiments), or (c) by grafting an additional primary vesicle into the prospective orbital mesenchyme on one side (15 experiments). The capacity of the trabeculae cranii and olfactory sacs to achieve normal morphogenesis has been studied by a series of isolations (45 experiments). Following removal of one primary optic vesicle, the orbit is reduced in size. Asymmetry of the head appears in the frontal region and later in the upper beak, and precedes the stage of trabecular fusion. The interorbital septum is only slightly smaller than normal. Following removal of both primary optic vesicles, both orbits are much reduced in size including the interorbital septum, which, however, is still fenestrated. The palpebral fissure is very small, but the three eyelids are recognizable at certain stages. The fronto-nasal process and its derivatives are displaced backwards so that the mandible’s growth is obstructed to such a degree that it catches on the upper beak and is doubled back on itself (stages 32 and 34). It is freed when the mouth opens widely, and thereafter always exceeds the upper beak in length. The nasal cavities remain close to their primitive position beneath the telencephalon so that the olfactory nerves are very short. The processus tectalis grows into the forehead. The planum supraseptale (anterior orbital cartilage) lies in close proximity to, but not in continuity with the posterior orbital cartilage. During the later stages of embryonic development the cephalic flexure is exaggerated. The trabeculae cranii fuse on schedule in the experimental anophthalmic embryo. A normal trabecula communis also forms when the appropriate prospective tissue is isolated in the coelom of another embryo. There is no evidence of trabecular contribution to the upper thin parts of the interorbital or nasal septa. Although eye size is correlated morphologically with trabecular pattern, there is no evidence that the eyes or orbits play an essential role in the mechanism of trabecular fusion in ontogeny. The convergent movement of the olfactory cavities does not occur to the normal degree without the presence either of the eye or of the orbital mesenchyme. When an additional optic vesicle is grafted on one side (homograft), a ‘composite eye’ forms in which the retinal elements of the graft and native eye are enclosed in a common scleral cartilage. There is a considerable increase in the size of the palpebral fissure and in the longitudinal dimension of the orbit (up to c. 50 percent.). The beak deviates towards the opposite side and the quadrate cartilage and otic capsule and optic lobes are displaced. The reciprocal influence of eye and orbit is well shown in these last experiments, for even though the orbit is considerably enlarged, the composite eye does not fully realize its potential growth capacity.