86 



ZOOLOGY 



SECT. 



and sometimes forms a thin layer over the enamel ; it has prac- 

 tically the structure of bone. At the inner end of the tooth there 

 is frequently an aperture (PH'} leading into a cavity (PH) filled 

 in the fresh condition by the tooth-pulp, a sort of connective- 

 tissue plug abundantly supplied with nerves and blood-vessels. 



In the development of a tooth (Fig. 769, B) the deep layer of the 

 buccal epithelium becomes invaginated and grows inwards into 

 the sub-mucosa in the form of a narrow cord, the enamel-organ 

 (SK). The distal end of this enlarges into a flask-like form, and 

 the bottom of the flask becomes invaginated (Ma) by the growth 



fe ME 



ZS 



FIG. 769. A, longitudinal section of a tooth, semi-diagrammatic. PH, pulp-cavity ; PIT, opening 

 of same ; Ztf, dentine ; ZC, cement ; ZS, enamel. B, longitudinal section of developing 

 tooth. By, submucosa ; DS, dentine ; Ma, invaginated layer of enamel-organ ; ME, epithelium 

 of mouth ; 0, odontoblasts ; SK, stalk of enamel-organ ; ZK, tooth-papilla. (From Wieders- 

 heim's Vertebrate.) 



of a conical process of the sub-mucosa, the dental papilla (ZK). 

 Mesoderm cells accumulate on the free surface of the papilla 

 and form a distinct layer of cells called odontollasts (0). From 

 these the dentine is formed in successive layers, which gradually 

 accumulate between the layer of odontoblasts and the inner or 

 invaginated layer of the enamel-organ. The lower, or proximal, 

 part of the papilla remains uncalcified and forms the tooth-pulp. 

 The enamel is formed by the deposition of successive layers of 

 calcific matter from the inner or invaginated layer of the enamel- 

 organ, the cement by the ossification of the tissue immediately 

 surrounding the papilla. Thus the tooth is partly of ectodermal, 

 partly of mesodermal, origin. 



