. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. THE SKULL. 59 While the first and the last of these are gradually surrounded by cartilage, and, especially in higher types, more and more drawn in to the skull proper, the lateral walls of the basal plate become raised up, and begin to grow round the brain on both sides, eventually extending even to the dorsal region. Thus a con- tinuous cartilaginous capsule is formed, such as persists through- out life in Elasmobranchs for example. But in by far the greater number of Vertebrates, the cartilage
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. THE SKULL. 59 While the first and the last of these are gradually surrounded by cartilage, and, especially in higher types, more and more drawn in to the skull proper, the lateral walls of the basal plate become raised up, and begin to grow round the brain on both sides, eventually extending even to the dorsal region. Thus a con- tinuous cartilaginous capsule is formed, such as persists through- out life in Elasmobranchs for example. But in by far the greater number of Vertebrates, the cartilage does not play so great a part, and is, as a rule, confined to the base and lower parts of the sides of the skull and to the sense-capsules, except in the occipital region, where it always extends over the brain. The rest of the skull, more particularly the roof, becomes directly converted from membrane into bone. Thus it may be stated generally, that the higher the systematic position of the animal, the less extensive are the cartilaginous constituents and the more important the bony. I. The Visceral Skeleton. The skeletal parts of the visceral arches, always formed in hyaline cartilage, encircle the anterior section of the alimentary canal, lying embedded in the inner part of the walls of the throat. FIG. 45.—DIAGRAMMATIC TRANSVERSE SECTION OF THE THIRD STAGE IN THE DEVELOPMENT OF THE PRIMORDIAL SKULL. 0, notochord ; Tr, trabeculoe, which enclose the brain (C) ventrally and laterally ; 0, auditory capsule ; RH, the cavity of the pharynx, enclosed by the visceral skeleton ; 1 to 4, the individual elements composing each visceral arch, which latter is united with its fellow by a basal piece (Cp). (Figs. 45 and 46, B,B). Always present in a greater number (up to as many as nine) in gill-breathing animals than in higher types, they gradually become reduced, so that in the Amniota the remains of the three or four anterior at most are seen: they further undergo a change of functi
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