TEETH 



IOI 



only produce the enamel but extend over the roots, so that they are de- 

 scribed as forming moulds for the teeth which develop within their con- 

 cavities. The tissue enclosed by the enamel organ is a dense mesen- 

 chyma, constituting the dental papilla. It becomes the pulp of the tooth, 

 and produces, at its periphery, the layer of dentine. As the tooth de- 

 velops, the connection between its enamel organ and the dental lamina 



Oral epithelium. 



Enamel 

 organs. 



Dental 

 groove 



Dental lamina. 



Papillae. 



Enamel organs. Necks of enamel organs. 

 ABC D 



FIG. 90. DIAGRAMS SHOWING THE EARLY DEVELOPMENT OF THREE TEETH. 

 (One of the teeth is shown in verticle section.) 



becomes reduced to a flattened strand or neck of epithelial tissue, which 

 subsequently disintegrates. 



In order to produce enamel organs for the three permanent molars, 

 which develop behind the temporary teeth on either side of the jaws, the 

 dental lamina grows backward, free from the oral epithelium. This 

 backward extension becomes thickened and then inpocketed by a papilla, 

 thus forming the enamel organ for. the first 

 permanent molar in embryos of 17 weeks (180 

 mm.) . It grows further back, and gives rise to 

 the enamel organ for the second molar at about 

 six months after birth, and for the third or late 

 molar (wisdom tooth) at five years. In rare 

 cases, several of which have been reported, 

 there is a fourth molar behind the wisdom tooth, 

 and it is assumed that in these cases the dental 

 lamina continued its backward growth beyond 

 the normal limits (Wilson, Journ. Anat. and 

 Physiol., 1905, vol. 39, pp. 110-134). 



The permanent front teeth develop from enamel organs on the labial 

 side of the deep portion of the dental lamina (Fig. 91). Owing to the 

 obliquity of the lamina the permanent teeth are on the lingual side of the 

 deciduous teeth. The enamel organs for the incisors develop slightly in 

 advance of those for the canines, but all of these are indicated in an embryo 

 of 24 weeks (30 cm.) described by Rose. He found the enamel organs 

 for the first premolars in an embryo of 29 weeks (36 cm.) and for the second 



QE. 



D.R. 



E.O. 



FIG. 91. TEETH FROM A HUMAN 

 EMBRYO OF 30 CM. (Modified 

 from R6se.) 



E. and E. O., Enamel organs of 

 a deciduous and of a perma- 

 nent tooth respectively; D. 

 R., dental lamina; O. E., oral 

 epithelium; P., papilla. 



