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  Section: Anatomy of Vertebrate Animals » Organisation of the Vertebrata Skeleton
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The General Modifications of the Vertebrate Skull


The lowest vertebrated animal, Amphioxus, has no skull. In a great many fishes, the development of the skull carries it no further than to a condition which is substantially similar to one of the embryonic stages now described; that is to say, there is a cartilaginous primordial cranium, with or without superficial granular ossifications, but devoid of any proper cranial bones. The facial apparatus is either incompletely developed, as in the Lamprey; or, the upper jaw is represented, on each side, by a cartilage answering to the palatopterygoid and part of Meckel's cartilage, while the larger, distal portion of that cartilage becomes articulated with the rest, and forms the lower jaw. This condition is observable in the Sharks and Rays. In other fishes, and in all the higher Vertebrata, the cartilaginous cranium and facial arches may persist to a greater or less extent; but bones are added to them, which may be almost wholly membrane bones, as in the Sturgeon; or may be the result of the ossification of the cartilaginous cranium itself, from definite centres, as well as of the development of superimposed membrane bones.


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