250 BULLETIN OF THE 



The fact that the dental lamina in the region of the second and third 

 incisors is directly continuous with the differentiated portion of the 

 plunging wall in the region of the first incisor, and has the same histo- 

 logical characteristics as that portion, seems good reason for believing 

 that the latter is the representative of the dental lamina, which in this 

 region never becomes further developed ; whereas in the region of the 

 second and third incisors it has become prolonged, and has changed its 

 direction, owing to the multiplication of its undifferentiated cells. 



In the canine region (fig. 20), the dental lamina at the place of its 

 connection with the plunging wall has become very narrow. Its width 

 at this jwint is very little more than the combined width of the two 

 malpighian layers which form its walls, the corneous layer of epithelium 

 being at this stage almost imperceptible. The deeper portion of the 

 lamina by its enlargement has given rise to an enamel germ, which, 

 although much smaller than the corresponding germ on the lower jaw, 

 has the same histological characteristics. 



The next stage studied was that of an embryo 87 mm. long. The 

 lamina in the region of the first incisor (fig. 21) seems neither to have 

 advanced nor to have retrograded in development from the condition in 

 the embryo 56 mm. long. In the region of the remaining incisors 

 (figs. 22, 23) the walls of the dental lamina have become thicker, and 

 the corneous layer of epithelium forming its centre has undergone the 

 same change as in the centre of the canine enamel germ of the preced- 

 ing stage, — the cells are smaller and less regular in shape In the 

 region of the third incisor (fig. 31) the differentiated portion of the 

 dental lamina, together with some of the unmodified portion, has begun 

 to separate from the buccal epithelium ; the mesodermic tissue by its 

 ingrowth occupies for some extent the region which in earlier stages 

 was uninterrupted epithelium. Although in this condition the lamina 

 has a comparatively broad connection with the plunging wall, sections 

 through the posterior portion of the incisor region show it entirely 



arranged. On account of this irregularity the layer is in some places broader than 

 in others, and presents an irregular outline on its mesodermic side. The labial 

 wall of the lamina is histologically unchanged from the malpighian layer of tiie 

 buccal epithelium. It is one cell deep, and has an even outline on the side toward 

 the mesoderm. 



The lingual wall increases in extent as the result of cell multiplication and forms 

 the base of the enamel organ. Since the cells of the base of the enamel organ 

 surround the dentine germ and at length produce the enamel, it is obvious that 

 the lingual wall of the lamina produces, first, the enamel organ • and finally, the 

 enamel. 



