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-iml]) , a sort of connective- 

 tissu3 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 cnamcl-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 



B 



Fic. 769. — A, longitudinal section of a tooth, semi-diagi-animutic. PH, pnlp-cavity ; PH", opening 

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

 tooth, lifi, submucosa ; DS, dentine ; if", invaginated Liyer of enamel-organ ; ME, epithelium 

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

 heim's Vcrtebratii.) 



of a conical process of the sub-mucosa, the dental 2^ccpilla {ZK). 

 Mesoderm cells accumulate on the free surface of the papilla 

 and foi-m a distinct la^-er of cells called odontohlasts (0). From 

 these the dentine is formed in successive layers, which graduall}- 

 accumulate between the layer of odontoblasts and the inner or 

 invaginated la3^er of the enamel-organ. TIic lower, or proximal, 

 part of the papilla remains nncalcifiod 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 immediatel}^ 

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

 partly of mesodermal, origin. 



