80 



ZOOLOGY 



SECT. 



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. 724, B) the deep layer of the 

 buccal epithelium becomes invaginated and grows inwards or 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 

 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 odontoblasts (0). From 



ZS 



zc 



zir-A 



FIG. 724. A, longitudinal section of a tooth, semi-diagrammatic. PH, pulpcavity ; PH', opening 

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

 tooth ; Bg, 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 Vertebrata.) 



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 

 calcine 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. 



In some Fishes the scales or elements of the dermal exo- 

 skeleton, pass insensibly into the teeth over the ridges of the 



