THE FCETUS. 921 



2. Teeth. These organs are developed in the interior of a cavity, 

 named the dental follicle or sac, by means of the elements of three germs : 

 one belonging to the ivory, another to the enamel, and a third to the 

 cementum. 



Follicle. The dental follicle is an oval cavity with walls composed of 

 two layers: the external is fibrous and complete; the internal, soft or 

 gelatinous, is allied at the bottom to the ivory germ. The latter is a 

 prominence which is detached from the bottom of the follicle, and has the 

 exact shape of the tooth. Its structure comprises, in the centre, delicate 

 connective tissue provided with vessels and nerves, and on the surface a 

 layer of elongated cells. At the summit of the follicle, facing the ivory 

 germ, is the enamel germ ; it is exactly applied to the dental pulp which it 

 invests like a cap, and is composed of a small mass of mucous connective 

 tissue covered by a layer of cylindrical cells, joined to the buccal epithelium 

 by the gubernaculum dentis. The cementum organ manifestly exists in the 

 Foal, according to M. Magitot. The base of the ivory germ has been found, 

 but it disappears rapidly after having performed its function. 



How are the different parts of the dental follicle developed ? On the 

 free borders of the maxillae the epithelium of the buccal mucous membrane 

 forms an elongated eminence the dental ridge. Below this ridge, the 

 epithelium constitutes a bud which develops in size, and is sunk in the 

 mucous membrane : this is the enamel germ ; it has a layer of cylindrical 

 cells on its deep surface, and in its centre are round cells. After a certain 

 time, it is only joined to the epithelium by a very thin line of cells the 

 gubernaculum dentis. While this enamel germ grows downwards, it covers, 

 by its base, a connective bud which rises from the mucous derma. The 

 two buds are reciprocally adapted to each other, and around them the 

 connective tissue condenses, and gives rise to the walls of the follicle. It 

 will therefore be seen, that the enamel organ is a dependency of the 

 epithelium, and the ivory organ a production of the mucous derma. 



Formation of the ivory, enamel, and cementum. The ivory and enamel 

 are developed by the modification of the elements situated at the surface of 

 their germ. 



It has been shown that the germ of the ivory, or dental pulp, had 

 exactly the form of the future tooth ; consequently the ivory which arises 

 from its periphery offers the shape of this tooth. The ivory (dentine) is 

 constituted by the cells of the germ, which elongate, send out prolongations 

 the dental fibres that ramify and anastomose, and by an intercellular 

 substance uhich is impregnated with calcareous matter, is moulded around 

 these fibres, and forms the dental canaliculi. 



The enamel is derived from the deep cells of its germ, which are 

 elongated and prism-shaped, and are calcified in becoming applied to the 

 surface of the ivory. 



The cementum is developed at the expense of the walls of the follicle, 

 according to the mode of ossification of the connective tissue. 



Eruption. As the ivory is formed, the tooth increases in length and 

 presses the enamel germ upwards; the latter, constantly compressed, 

 becomes atrophied, and finally disappears when the tooth has reached the 

 summit of the follicle. In the same way the young organ pierces the 

 dental follicle and gum, and makes its eruption externally. 



Such is the mode of development of the caducous teeth, and the per- 

 manent ones are formed in the same manner. There is seen, during the 

 development of the enamel germ of the temporary tooth, a bud detaching 



