SKULL. We may distinguish in the skull the cranium or braincase and the visceral skeleton. In the development of the human skull three stages may be distinguished which correspond with phylogenetic stages: (1) The fibro-eonnec tive tissue stage. This is represented in pity logeny by the condition in Amphioxus, where a fibrous cordal sheath surrounds the notochord.
(2) The cartilaginous stage. In the anterior region of adult selachians a large cartilaginous capsule, open above, completely surrounds the brain below and laterally, derived from two pairs of cartilage plates. Ventral to the skull the visceral skeleton arises, consisting of the upper and lower jaws and the six branehial arches, the foremost of which early differentiated itself from the other five, entered into connection with the lower jaw, and constitutes the hyoid arch. The lower jaw arises in a manner precisely equivalent to a typical gill-arch, and is composed of two pieces on each side, the quadrate and Meckel's cartilage. Very early a forward outgrowth from the quadrate gives rise to the upper jaw. (3) The
bony stage is represented in the bony ganoids, where the frame-case is covered by enamel plates. Dermal hones also cover over the branchial arches and gills, forming the `opercultun.' Even in the Amphibia the bones of the skull preformed in cartilage can he artificially separated from der mal bones, but the higher we go in the vertebrate scale the more intimate Is the union, until in mammals the two bones are developed at the same time and are inseparably fused in the adult. With the loss of gills goes that of the opercular apparatus, and the cranium becomes more com pact. Of the branchial there remains the first, the mandibular, the second, the hyoid, and a part of the third, which fuses with the hyoid. Finally the axis of the cranium curves. The curve is first considerable in reptiles and birds and reaches its maximum in man. Sae SKtiLL; for the anatomy of the bones of the ear, see EAR; and for that of the dental appara tus, see TEF:T II