. The Cambridge natural history. Zoology. VERTEBRAL COLUMN 2or compared with the basi-dorsals they Take but a small share in forming the walls of the neural canal. Well-developed but somewhat fragmentary inter-ventrals are present. The haemal arches and spines are formed by the downgrowth and ventral union of the basi-ventrals as in the Dog-Fish, and apparently without the aid of costal elements. In Polyoclon the ribs are vestigial,-' but in Acipenser they are well developed. The neural arches and spines, and their haemal representatives in the tail, and also the ribs, are partially ossified,
. The Cambridge natural history. Zoology. VERTEBRAL COLUMN 2or compared with the basi-dorsals they Take but a small share in forming the walls of the neural canal. Well-developed but somewhat fragmentary inter-ventrals are present. The haemal arches and spines are formed by the downgrowth and ventral union of the basi-ventrals as in the Dog-Fish, and apparently without the aid of costal elements. In Polyoclon the ribs are vestigial,-' but in Acipenser they are well developed. The neural arches and spines, and their haemal representatives in the tail, and also the ribs, are partially ossified, or ensheathed by bone. In the existing Crossopterygii, Holostei, and Teleostei, popularly Fig. 116.—Diagram to illus- trate the groii2:)iiig of vertebral elements to form vertebrae, A, in an Elasmo- braneh, B, in Amia, and C, in Lepidostevs. ^ Basi-dorsals; , basi- ventrals; , inter- dorsals ; /. T', inter-vent- rals ; , inter-verte- bral cartilage divided by a concavo - convex cleft ; , precentrum ; j:), postcentrum. The square blocks represent indi- vidual vertebrae, and the oblique lines, the attach- ments of the myocommata. known as the " bony Fishes," the vertebral column assumes a more familiar character, and at the same time we meet with interesting illustrations of the different methods by which the separate com- ponent vertebral elements of the more primitive types of " back- bone " are concentrated together in groups, and fused to form that complex physiological product, the complete bony vertebra.^ In most of these Fishes each vertebra is formed by the aggregation and fusion of a pair of basi-dorsals and a pair of basi-ventrals, and includes, in addition, a pair of inter-dorsals, which may either be the pair in front of the basi-dorsals or the pair behind^ and also a pair of inter-ventrals, which, similarly, may be tha 1 Bridge, 1897, p- 722. ^ Gadow, op. cit. p. 201, et. Please note that these images are extra
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
