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 
conncction with the plunging wall has become very narrow. Its width 
at this point 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 the 
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. 
