DIGESTIVE TISSUES 



293 



cells are destroyed as the crown of the tooth, supplied with its enamel, 

 emerges from the dental groove. 



With the evolving of this enamel tissue the papilla grows and as- 

 sumes the shape of a mature tooth (Figs. 256 and 257). The mesen- 

 chyme becomes differentiated into an inner and an outer zone of cells. 

 The inner zone gives rise to the mass of connective-tissue elements 

 known as the pulp. This pulp contains stellate connective-tissue cells, 

 connective-tissue fibrils, and a semifluid inter-fibrillar substance. The 

 pulp supports the nerve and blood supply of the tooth. The outer 

 zone of mesenchymal cells becomes pyriform, with their small ends radiat- 

 ing from the axis of the tooth (Fig. 258). These elaborate at their 

 distal ends the hard part of the tooth known as the dentine. They are 

 called odontoblasts. Numerous fine canals traverse the layer of dentine. 



FIG. 257. Half-matured tooth of same animal; blood well into the papilla, whose outer edge is 

 hardening into the dentine, en., enamel laid down by the tall columnar epithelium, x, plane 

 of section shown in Fig. 258. X 55. 



Into these canals or canaliculi the odontoblasts send protoplasmic pro- 

 cesses. These processes take part in the calcifying of the dentine. This 

 calcification takes place first at the periphery where the processes end. 

 As calcification advances the protoplasmic processes of the odonto- 

 blasts retreat, and the canaliculi are almost obliterated. The root of 

 the dogfish's tooth is not developed to form a fang. It is a perforated 

 plate rather than a tube. It is composed of a dentine wall without 

 enamel and a pulp cavity. The nerve and blood supply enters the 

 tooth through this root. 



Accessory digestive glands. Calcium-carbonate glands. These 

 glands occur in the earthworm. Externally they appear as three pairs 

 of pouches or diverticula from the side of the oesophagus. The first 

 pair of diverticula lie in the tenth segment. These are the most promi- 

 nent. The second and third pair lie in the eleventh and twelfth seg- 

 ments respectively. In section the first pair is clearly seen to be an 



