IOO 



HISTOLOGY 



The Development of the Teeth. The first indication of tooth develop- 

 ment in human embryos is a thickening of the oral epithelium, which 

 has been observed in specimens measuring 11-12 mm. At this stage 

 the oral plate, which marks the boundary between ectoderm and ento- 

 derm, has wholly disappeared, but it is evident that the thickening takes 

 place in ectodermal territory. The tongue is well developed, but the 

 upper and lower lips are not as yet separated by depressions from the 

 structures within the mouth. Soon after the thickening has appeared, it 

 grows upward in the upper jaw, and downward in the lower jaw, into the 

 adjacent mesenchyma, thus forming an epithelial plate which follows the 

 circumference of either jaw. It undergoes the same sort of transformation 

 in both the maxilla and mandible, and the following description of the 

 conditions in the mandible is therefore applicable to both. As the plate 

 descends into the mesenchyma, it divides into a labial lamina in front, 



a b c d 



FIG. 89. SAGITTAL SECTION THROUGH THE TONGUE AND LOWER JAW OF A HUMAN EMBRYO OF 22 MM 



X 20. 



a, Labial lamina; b, dental lamina; c, Meckel's cartilage; d, tongue. 



which brings about the separation of the lip from the gum, and a dental 

 lamina behind, which is concerned with the production of the teeth (Fig. 

 89). At first the dental lamina is inclined decidedly inward or toward 

 the tongue, as seen in the figure, but later it descends from the oral epithe- 

 lium almost vertically. Taken as a whole it is a crescentic plate of cells 

 following the line of the gums, along which the teeth will later appear. 



The further development of the dental lamina is shown diagrammatic- 

 ally in Fig. 90, A-D, each drawing representing a part of the oral epithe- 

 lium above and dental lamina below, freed from the surrounding mesen- 

 chyma. The labial side is toward the left and the lingual side toward 

 the right. Almost as soon as the dental lamina has formed, it produces a 

 series of inverted cup-shaped enlargements along its labial surface (Fig. 

 90, B), and these become the enamel organs. There is a separate enamel 

 organ for each of the ten deciduous teeth in either jaw, and they are all 

 present in embryos of two and one-half months (40 mm.). They not 



