STRUCTURE AND GROWTH OF INCISOR TEETH 67 
in these two parts. On the labial side at the anterior end, the 
organ has advanced to the condition where functional activity 
is beginning, while the oral side has remained stationary, or has 
actually retrogressed. Thus in the innermost layer on the labial 
side of the lower incisor, where the ameloblasts have begun to 
form enamel, these cells measure 30 to 34u in length, while the 
non-functional cells on the oral side of the innermost layer are 
low columnar or cubical in shape and measure only 12, in length 
(fig. 13). Comparison of these measurements with those at 
19 days shows that the cells of the inner layer of the labial side 
of the enamel-organ have advanced in length from 24u to 30: or 
34u, while the cells on the lingual side have decreased from 20 to 
12u. There is, therefore, a primary tendency for the cells of the 
inner layer to develop equally in all parts, but very soon the non- 
enamel-forming cells of the lingual side begin to retrogress, 
while the functional cells of the labial side continue to grow. 
This constitutes another point of contrast with the development 
of the crowns of rooted teeth. For here in the 21-day fetus, 
when the enamel and dentine formation has just begun, these 
substances are thickest, not over the apex of the tooth-forming 
organs, as in the usual method, but at a short distance posterior 
to this point, on the labial surface. Thus, not only are the odon- 
toblasts and the ameloblasts first differentiated on the labial 
side, posterior to the apex, but at this region enamel and dentine 
formation is also evidently first begun. 
Over the apex of the dental papilla there is apparently a very 
thin outline of dentine deposited, but within this, in the tissues 
of the apex of the dental papilla, there is also beginning an irregu- 
lar formation of a hard matrix. Between the cells of the pulp, 
trabeculae of a bone-like material are appearing. As develop- 
ment proceeds this substance increases until the final result is, 
as seen in figure 20, that the primary apex of the tooth has a 
bone-like structure, consisting of cells imbedded in lacunae 
within a dense matrix. This has been called by Tomes (’04) 
‘osteo-dentine.’ 
A similar difference between the labial and oral sides is noted 
in the cells on the margin of the dental papilla, which are be- 
