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 they grow deeper into the rnesoblast become bell-shaped, and 

 enclose modified masses of connective-tissue, the dental papilla'' ; 

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

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

 nective tissue germs come into the closest relation with one another 



FIG. 191 A. DIAGRAM OF THE DEVELOPMENT OF A TOOTH. 



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

 brana adamantina of enamel-organ ; 0, odontoblasts ; DS, dentine; By, Bg, 

 connective tissue follicle or sac surrounding the tooth. 



FIG. 191i5. SEMIDIAGRAMMATIC FIGURE OF A LONGITUDINAL SECTION THROUGH 



A TOOTH. 



ZS, enamel ; ZB, dentine ; ZC, cement ; PA 1 , aperture of the pulp-cavity (PH). 



and give rise respectively to the calcified enamel and dentine (ivory'), 

 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 



