70 W. H. F. ADDISON AND J. L. APPLETON, JR. 
of the tooth has apparently advanced somewhat into it. So 
this thick stratified layer of epithelium forms a close-fitting 
investment about the tooth apex, and is continuous posteriorly 
with the remainder of the enamel-organ. But in this epithelial 
cap there are no ameloblasts and consequently there can be no 
enamel over the osteodentine which forms the tip of the primi- 
tive tooth. This substance forming the tip of the unerupted 
tooth is a form of secondary dentine with its cells located in 
the lacunae of the matrix. Passing backwards, one comes to 
the ordinary dentine containing the vascular pulp with odonto- 
blasts situated at the periphery of the pulp-chamber in a regular 
manner. 
As the odontoblasts were first differentiated labially, and 
dentine formation began there before on the other side, the den- 
tine of the labial side is thicker than on the lingual side. Thus 
at a point about the middle of the entire tooth structure, the 
dentine measured 54u labially and 20u orally (fig. 15). Be- 
tween the odontoblasts are numerous fine capillary loops. At 
this region may also be seen the characteristic structure of the 
enamel-organ (fig. 15). This extends all around the tooth, 
but is much thicker on the labial side than elsewhere. This 
difference in thickness is seen in all the constituent layers. In 
the inner layer, the tall ameloblasts of the labial surface measure 
40u, while the similarly situated cells on the other surfaces are 
cubical and measure only 10u. Comparing these with the 
previous stage described, it is seen that the cells on the labial 
surface have increased and those on the other surfaces have 
decreased. Of the middle layer on the labial side, the stratum 
intermedium is a distinet line of cuboidal cells, one to two rows 
in thickness, lying behind the ameloblasts. The other con- 
stituent—the original enamel pulp—is small in amount and is 
principally within the elevations of the outer layer, which form 
the beginning of the epithelial papillae. The cells of the outer 
layer, somewhat irregular in shape with round nuclei, are in a 
single row. Between the developing papillae (called by Sachse 
Stiitzpapillen) are numerous capillary blood-vessels. On _ the 
other surfaces, practically nothing remains of the middle layer, 
