. Chordate morphology. Morphology (Animals); Chordata. vascular canal Shorpey's fibers canals of Williamson B Figure 8-37 Rhomboid ganoid scales of lepisosteus. A, outer view of scole with growth increments indicated by dashed lines; B, longitudinal section through a scale; C, histology of posterior margin of a scale. (After Nickerson, 1893; Kerr, 1952; Rauther, 1927-1940) probably nothing more. The processes of the flattened osteoblast of Lepisosteus extend out in a single interlamellar plane, while those of Amia radiate in all directions. Teleost The teleost may have laminated scales covered
. Chordate morphology. Morphology (Animals); Chordata. vascular canal Shorpey's fibers canals of Williamson B Figure 8-37 Rhomboid ganoid scales of lepisosteus. A, outer view of scole with growth increments indicated by dashed lines; B, longitudinal section through a scale; C, histology of posterior margin of a scale. (After Nickerson, 1893; Kerr, 1952; Rauther, 1927-1940) probably nothing more. The processes of the flattened osteoblast of Lepisosteus extend out in a single interlamellar plane, while those of Amia radiate in all directions. Teleost The teleost may have laminated scales covered with a generally noncellular bony material called hyoden- tine (Figure 8-39). In Callichthys the hyodentine contains canals for protoplasmic extensions of cells lying in the large vascular channels. The hyodentine is also penetrated by Sharpey's fibers. The scales may be associated with spine- like denticles like those observed in Lepisosteus or Polyplerus. These have an enamel cover and a dentine interior sur- rounding a pulp cavity. The denticles are attached to the scale by ligaments or embedded in the hyodentine. Below the hyodentine is laminated bone or cell-containing, fibrous connective tissue which may be partly calcified. The scale of the teleost is of a cycloid type with circum- ferential growth rings (Figure 8-40). In many species, there are tooth-like extensions from the posterior margin; this modification identifies the ctenoid scale. EMBRYOLOGiCAL DEVELOPMENT The scales of Polypterus develop in the outer parts of the dermis, where knots of fibroblasts gather to form buds (Figure 8-41). The outer part of this cell aggregation forms the scale sac in which two layers of osteoblasts appear. The two layers become sepa- rated by an imperforate lamina of bone. To this initial lamina, which disappears, spongy bone is added above and below. The outer layer of enamel and dentine is now applied along with a lamina of bone to the under surface. The outer face of the scale li
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