The development of the orbital region of Caretta caretta (Chelonia, Reptilia).

In the development of the orbital region of the Caretta chondrocranium, the rearrangement of the several eye muscles seems to be correlated with the apparent anterior shift of the neurocranial element, the pila metoptica. The pila consists of the main part, the supratrabecular cartilage, and five processes, the superior, middle, anteromedial, antero-inferior and the postero-inferior. The superior process forms the attachment of the superior rectus muscle and, together with the muscle, moves anteriorly during development. In the course of the anterosuperior shift of the inferior rectus muscle, the antero-inferior process degenerates and the anteromedial process is newly formed. The postero-inferior process gives origin to the posterior rectus muscle and regresses as a result of the upward shift of the muscle. All these changes are the result of the secondary arrangement of the eye muscles gathered around the secondary optic foramen which has been newly formed through the pila metoptica deformation. The elevation of the optic nerve which is brought about through the formation of the interorbital septum, is another factor that brings about the above changes. Because of these changes, the anterior part of the cranial flexure, the pila metoptica, lies in a longitudinal plane and consequently it, as well as the cavum epiptericum, is obliterated and a large antero-inferiorly opening extracranial space, the orbit of the reptile, is produced.