A treatise on zoology . sed hy a multitude ofminute nearly parallel canaliculi (dentinal tubules) with delicatebranches. No cells are included in the dentine, but fine processesof the odontoblasts extend up the canaliculi (Figs. 79, 81). The CHONDRICHTH YES first appearance of dentine takes place immediately Ijeneath theepidermis, in direct continuity with the basement meml)rane, ofwhich it may be considered to be a thickening (Fig. 79, A-D). Itis secreted by a group of mesoblastic cells (odontoblasts), the rudi-ment of the future dental papilla or pulp. Dentine differs frombone not only in th
A treatise on zoology . sed hy a multitude ofminute nearly parallel canaliculi (dentinal tubules) with delicatebranches. No cells are included in the dentine, but fine processesof the odontoblasts extend up the canaliculi (Figs. 79, 81). The CHONDRICHTH YES first appearance of dentine takes place immediately Ijeneath theepidermis, in direct continuity with the basement meml)rane, ofwhich it may be considered to be a thickening (Fig. 79, A-D). Itis secreted by a group of mesoblastic cells (odontoblasts), the rudi-ment of the future dental papilla or pulp. Dentine differs frombone not only in that it contains no cells, but also in that it growson one surface only—the surface next to the dentinal pulp. Asa rule, the base of the dentine cone spreads inwards into theunderlying connective tissue, and may change in structure fromtrue dentine to a looser kind of trabecular calcified tissue. A basal plate is thus formed, Avhich tends to cut off the pulp-cavity below, leaving only one or two narrow openings Avhereby. Fig. so. Jledian dorsal denticle of Ttfya hlaiula. A, left-side view. 13, section much enlarged, h,basal x^late ; d, deuliiial tubules ; o, opening of pulp-cavity, ; , projecting spine ; tr,modified trabecular dentine. the blood-vessels, nerves, and lymph-chainiels can j^ass distal region of the denticle breaks through the epidermis andemerges freely on the surface. It is covered Avith a shiny enamel-like layer. Of the real nature of this layer observers are stilluncertain. According to Leydig and Rose [372], it is a specialouter zone of vitrodentine, with very fine canaliculi. It doesnot appear to be true enamel, secreted by the epidermal cells, suchas is found on the teeth of higher vertebrates. Tomes [439]believes it to be formed by the combined action of the epidermiswithout and the odontoblasts within. We may here briefly describe the chief varieties of dentine foundin fish (Tomes [438, 440], Kose [372], Owen [311rt]). They maybe classif
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