120 



CHONDR1CHTH YES 



first appearance of dentine takes place immediately beneath the 

 epidermis, in direct continuity with the basement membrane, of 

 which it may be considered to be a thickening (Fig. 79, A-D). It 

 is secreted by a group of mesoblastic cells (odontoblasts), the rudi- 

 ment of the future dental papilla or pulp. Dentine differs from 

 bone not only in that it contains no cells, but also in that it grows 

 on one surface only the surface next to the dentinal pulp. As 

 a rule, the base of the dentine cone spreads inwards into the 

 underlying connective tissue, and may change in structure from 

 true dentine to a looser kind of trabecular calcified tissue. A 

 'basal' plate' is thus formed, which tends to cut off the pulp- 

 cavity below, leaving only one or two narrow openings whereby 



B 



FIG. so. 



Median dorsal denticle of Raja tlamla. A, left-side view. B, section much enlarged. 6, 

 basal plate ; d, dentinal tubules ; o, opening of pulp-cavity, p.c ; s.p, projecting spine ; tr, 

 modified trabecular dentine. 



the blood-vessels, nerves, and lymph-channels can pass through. 

 The distal region of the denticle breaks through the epidermis and 

 emerges freely on the surface. It is covered with a shiny enamel- 

 like layer. Of the real nature of this layer observers are still 

 uncertain. According to Leydig and Rose [372], it is a special 

 outer zone of vitrodentine, with very fine canaliculi. It does 

 not appear to be true enamel, secreted by the epidermal cells, such 

 as is found on the teeth of higher vertebrates. Tomes [439] 

 believes it to be formed by the combined action of the epidermis 

 without and the odontoblasts within. 



We may here briefly describe the chief varieties of dentine found 

 in fish (Tomes [438, 440], Rose [372], Owen [31 la]). They may 

 be classified as follows : typical dentine, with numerous canaliculi 

 radiating from a central pulp-cavity (Fig. 79); plicidentine, 



