Development of the orbital region in the chondrocranium of Caretta caretta. Reconsideration of the vertebrate neurocranium configuration.

By studying the development of the orbital region in the Loggarhead turtle (Caretta caretta) and some placental mammals, it has become clear that the orbital region of the neurocranium should not be regarded as merely a "bowl" to contain the brain, but rather that its ventral part is originally flexured along with the cephalic flexure of the neural tube. At this flexure, the neurocranium is to be divided into 2 parts, the anterior and posterior. The anterior part of the neurocranial sheet is medially perforated by the infundibulum and gives rise to pila metoptica laterally. The post orbital cartilage represents the posterior part. From the above "Bauplan" of the neurocranium, the following conclusions can be drawn: (1) the simple homology of the reptilian and placental mammalian pila metoptica is questionable; (2) the pila antotica is produced by the absorption of the mid-dorsal part of the postorbital cartilage, while the dorsum sellae in mammals is produce by the chondrification of the middle part of the same anlage; (3) homology of the ala hypochiasmatica in mammals with the supratrabecular cartilage in reptiles is more feasible than with the cartilago hypochiasmatica; and (4) the crista sellaris in reptiles is not a part of the primary cranial wall but probably of secondary production.