DENTINE 



119 



bases of neighbouring denticles may fuse; in this way have probably 

 arisen the dermal plates sometimes found in fossil Chondrichthyes 

 (p. 168). The placoid scales (Williamson [496a], Hertwig [211], 

 Klaatsch [264]) are essentially hollow cones of dentine surround- 

 ing a pulp-cavity. Dentine, which forms the bulk of the hard 



Fig. 70. 



Successive stages in the development of the denticles of the Dogfish, Saylli-u/m canicula, L. 

 In A is seen the first gathering of the soleroblasts, sc, below the basement membrane, b.m ; in 

 C, the first deposition of dentine, d (black) ; in E, three stages (from right to lei t) in the 

 formation of the basal plate, b.p. et, connective-tissue deep layer of cutis; c, "enamel" 

 (white); ml, modified epidermal cells; p, pulp-cavity. A-D, transverse, E, longitudinal 

 section, enlarged. 



tissue of the teeth of all Gnathostomes, is of mesoblastic orgin. 

 It is secreted by the odontoblasts lining the pulp-cavity, and 

 consists of a hard calcified matrix traversed by a multitude of 

 minute nearly parallel canaliculi (dentinal tubules) with delicate 

 branches. No cells are included in the dentine, but fine processes 

 of the odontoblasts extend up the canaliculi (Figs. 79, 81). The 



