240 



COMPARATIVE ANATOMY 



in the form of superficial papillae of the mucous membrane ; but 

 secondarily, owing to want of space, the epithelium of the mouth 

 grows inwards so as to give rise to a dental lamina which becomes- 

 enlarged distal ly at certain points to form the so-called enamel-organs^ 

 These as theygrow deeper into the mesoblast become bell-shaped, and 

 enclose modified masses of connective-tissue, the dental jpapillm; 

 the upper cells of the papillae, i.e., those next to the enamel-organ 

 are known as odontoblasts (Fig. 191, A). The epithelial and con- 

 nective tissue germs come into the closest relation with one another 



Fig. 191a. — Diagram of the Development of a Tooth. 



EM, epithelium of mouth ; SK, dental lamina ; ZK, dental papilla ; Ma, mem- 

 brana adamantina ol enamel-organ ; O, odontoblasts ; J)S, dentine ; Bg, Bg, 

 connective tissue follicle or sac surrounding the tooth. 



Fig. 191i;. — Semtdiacbammatic Figure of a Longitudinal Section through 



A Tooth. 



ZS, enamel ; ZB, dentine; ZC, cement; PA^, aperture of the pulp-cavity {PH). 



and give rise respectively to the calcified e^iamel and dentine {ivorj), 

 of which the teeth are composed. The enamel is the harder and 

 contains little organic matter, and the dentine is permeated by a 

 system of fine canals in which are delicate processes of the odonto- 

 blasts. A third, bone-like substance, the cement, is also formed 

 round the bases of the teeth, and between the folds of enamel 

 when these are present ; it may unite with the bones of the jaw. 

 The root of the tooth, embedded in the gums, is provided at its 



