40 Basic Structure of Vertebrates 



the two laminas are carried apart, one extending outward (labially), 

 the other inward (lingually). The labial lamina produces the first or 

 "milk-teeth," the lingual lamina, the later permanent teeth 

 (Fig. 35B, C). 



Early in the development of the labial lamina, a series of bell- 

 shaped enlargements, 10 in the upper and 10 in the lower jaw, appear 

 along its labial edge (Figs. 35B, 36). Each of these enamel-organs, 

 all present in a 10-week embryo, secretes the enamel of one of the 20 

 milk-teeth. The concavity of each enamel-organ is occupied by a der- 

 mal papilla consisting of loosely arranged mesodermal cells (mesen- 

 chyme). Adjacent to the enamel-organ, these cells form a layer which 

 secretes dentine at the surface of the layer next to the enamel -organ 

 (Figs. 35B, C, 37). Thus the dentine, secreted outward by the deep 

 mesodermal papilla, and the enamel, secreted inward by the ecto- 

 dermal layer, are deposited one against the other as they are destined 

 to be in the wall of the tooth. The dentine-producing cells (odonto- 

 blasts) become the permanent odontoblast layer lining the pulp- 

 cavity. The remaining cells of the papilla give rise to the connective 

 tissue and blood-vessels of the pulp. As development progresses, each 

 tooth-germ recedes from the dental lamina, but retains a temporary 

 connection with it by means of a narrow cord of cells (Figs. 35C, 36). 



Soon after the enamel-organs take form, each becomes differenti- 

 ated into three layers (Fig. 35C) : an inner ameloblast layer whose 

 cells produce the enamel; an outer layer known as the "outer enamel 

 layer," although it does not secrete enamel; and, between them, a mass 

 of loosely arranged ectodermal cells. The enamel-secreting (amelo- 

 blast) cells (Fig. 37) are derived directly from the stratum germinativ- 

 um of the ectodermal epithelium. Each ameloblast has the form of a 

 hexagonal prism and it secretes, at its end which is adjacent to the 

 odontoblast layer, a simple hexagonal solid prism of enamel. As the 

 thickness of the enamel layer increases, multiplication of ameloblast 

 cells increases the number of enamel-prisms. While the layer of enamel 

 grows by addition to its external surface, the dentine increases at its 

 deeper surface. Consequently, as the tooth develops, the ameloblast 

 and odontoblast layers move farther and farther apart (Figs. 35, 37). 



The lingual lamina of each jaw, ultimately losing connection with 

 the germs of the milk-teeth, forms a set of 16 enamel-organs lying in a 

 row parallel to and on the lingual side of the row of "milk "-germs. 

 Thus the germs of the 32 permanent teeth come to lie embedded in the 

 connective tissue of the jaws on the lingual side of the primary set 

 (Figs. 35C, 38, 39, 40). The permanent teeth, however, are relatively 

 slow in development, the most posterior molar usually not forming be- 

 fore the fifth year of childhood. 



