TOOTH-DEVELOPMENT. 141 



The Tooth-Sac. Coincidently with the development of the enamel- 

 organ and the growth of the dental papilla, the surrounding mesoderm dif- 

 ferentiates into a connective tissue envelope known as the dental or tooth-sac. 

 The latter not only closely invests the enamel-organ, but is intimately related 

 to the base of the dental papilla. In contrast to the epithelial enamel-organ 

 which is entirely without blood-vessels, the inner part of the tooth-sac is 

 abundantly provided with capillaries and is, therefore, an important source 

 of nutrition for the growing dental-germ. The part of the sac opposite the 

 root of the young tooth at first is prevented from coming into contact with 

 the dentine by the double layer interposed by the epithelial sheath. This 

 relation continues until the cementum begins to develop, when the vascular 

 tissue of the dental sac breaks through the epithelial sheath to reach the 

 outer surface of the dentine, upon which the cementum is deposited. In 

 consequence of this invasion the epithelial sheath is broken up into small 

 groups or nests of cells that persist for a long time as the epithelial 

 islands encountered within the fibrous tissue of the alveolar periosteum, 

 into which the dental sac is converted. As development proceeds, the 

 tissue of the tooth-sac becomes denser, the part opposite the root persisting 

 as the pericementum, while the superficial part blends with the tissue 

 forming the gum. 



The formation of the cementum is brought about through the 

 agency of mesodermic tissue in a manner almost identical with the develop- 

 ment of subperiosteal bone (page 47), the cement-producing cells, the 

 cementoblasls , corresponding to osteoblasts, and like them bringing about a 

 deposit of osseous matrix upon the osteogenetic fibres from the alveolar peri- 

 osteum. The cementum appears first in the vicinity of the neck of the tooth 

 and thence progresses towards the apex of the root, as the dentine of the fang 

 is formed. The layer of cementum is thickest at the apex, which it invests 

 except where the canal or canals remain for the blood-vessels and nerves 

 that pass to the pulp-cavity. 



Provision for the development of the permanent teeth is made by 

 the early differentiation of a second set of dental rudiments during the 

 growth of the first. This includes the outgrowth of the enamel-organs of 

 second dentition from the dental ledge and the subsequent appearance of 

 new dental papillae from the mesoderm. The enamel-organ for the first per- 

 manent molar appears about the seventeenth week of foetal life and is soon 

 followed by the corresponding dental papilla. The germs of the permanent 

 incisors and canines, including the papillae, are formed about the twenty-ninth 

 week and those for the premolars about one month later. The enamel- 

 organ of the second permanent molar appears about four months after birth 

 and the papilla about two months later, while the enamel-sac for the third 

 molar, which forms about the third year, precedes its papilla by almost two 

 years. It is evident, therefore, that the development of these teeth proceeds 

 very slowly, the embryonal structures being present years before the eruption 

 of the permanent teeth. The presence of the milk teeth and of the germs of 

 the permanent ones results in excessive crowding in the jaws during the fifth 

 year. In order to accommodate the representatives of both sets, the crowns 

 of the permanent teeth press between and against the roots of the milk teeth, 

 which then undergo absorption. The latter process is effected by connective 

 tissue cells, the odontoclasts, in a manner similar to that by which bone is 

 removed by the osteoclasts. In consequence, before the temporary tooth is 

 displaced it often is reduced to little more than the crown. Not until some 

 time after eruption are the roots of the permanent teeth fully formed. 



