STRUCTURE AND GROWTH OF INCISOR TEETH ai 
the cells form three layers, inner, middle and outer. The inner 
and outer layers, made up of columnar elements, stain more 
darkly than the middle layer, and the inner is thicker than the 
outer. The middle layer consists of elongated cells with oval 
nuclei, arranged for the most part with their long axes parallel to 
the surface of the enamel-organ. Frequent mitoses are also to 
be seen here, especially in the inner layer. 
In the region about 0.6 mm. anterior to this (fig. 19, c), where 
enamel formation has not yet begun, the innermost layer shows 
a single row of distinct tall columnar cells, the ameloblasts. 
The middle layer now shows two subdivisions (a) two or three 
layers of compacted flattened cells lying against the ameloblasts, 
and composing the stratum intermedium, and (b) a somewhat 
thicker stratum, lightly staining, of more loosely arranged cells, 
constituting the enamel pulp. The outermost layer is a single 
row of cubical cells, which form a straight continuous surface 
for the enamel-organ. Beyond this layer and in contact with 
it are numerous small blood-vessels. Passing still farther for- 
wards, the outermost layer becomes more sinuous in outline, 
and blood-vessels occupy the depressions between the elevations. 
This arrangement shows the beginning formation of the typical 
epithelial papillae. 
Seven days old 
At 7 days the tip of the tooth is in the oral epithelium (fig. 
20), and ready for eruption, being separated from the outside by 
only a thin layer of superficial cornified epithelium. The epithe- 
lial tissues immediately about the apex of the tooth show the 
appearance of pressure atrophy. The cell boundaries are more 
indistinct than elsewhere, the tissue takes the acid stain deeply, 
and there is increased granularity—evidently degenerative effects 
due to the pressure of the advancing tooth. 
In the upper jaw, the basal end of the tooth in its backward 
growth has reached the region of the maxilla, into which it 
continues to grow, pushing before it a little pocket of thin bone. 
The average length of the upper teeth at this age is 5 mm., and 
of the lower teeth, 7 to 8 mm. Their pointed apices, and their 
comparatively slight curvature are shown in figure 24. 
