STRUCTURE AND GROWTH OF INCISOR TEETH ta 
ameloblasts measure about 40u. These are backed by two rows 
of darkly staining flattened cells composing the stratum inter- 
medium. Next to these is the looser arrangement of stellate 
cells, comparable to the enamel pulp of ordinary tooth develop- 
ment, but with much smaller spaces between the cells. This 
tissue is covered by the layer of cells constituting the outer 
layer of the enamel-organ, and the two together constitute the 
epithelial papillae. At the summit of each of these papillae 
the cells of the outer layer are grouped in a more compact manner. 
With higher magnification processes can be seen running from 
the ameloblasts into the developing enamel—the so-called enamel 
processes of Tomes. 
At the basal formative part of the enamel-organ the three 
original layers show distinctly. At the thickened basal margin 
of the enamel-organ, around its entire circumference, is a mass 
of rapidly dividing cells. As seen in figure 16 this thickened 
margin is more noticeable on the labial side. Its peripheral 
zone as seen in longitudinal sections is deeply staining and its 
cells, more or less columnar in shape, are compacted together. 
The interior, of more lightly stained appearance, is composed 
of oval or elongated cells, irregularly parallel, but more loosely 
arranged than the cells of the periphery. This region consti- 
tutes the site of origin of the cells of the ever-forming enamel- 
organ. From this pass forward the outer and inner layers, and 
between them, in larger quantity than is found more anteriorly, 
the tissue of the middle layer. This for a short distance is all 
enamel pulp and shows no differentiated layer of stratum inter- 
medium. 
In this formative region on the labial side, the inner layer 
consists of columnar cells, the future ameloblasts, in which many 
mitoses are seen. While the outer layer consists of cells which 
are columnar near the margin, a short distance anterior to this 
(150u) they change shape, first to cubical, then to flattened 
cuboidal. Between the two layers are cells representing the 
enamel pulp. At this region there are no papillae, although 
numerous blood-vessels are seen alongside the outer layer of the 
enamel-organ. About 0.5 mm. from the basal end this outer 
