STRUCTURE AND GROWTH OF INCISOR TEETH 63 
At 15 days these thickenings have become more definite, 
and in the lower jaws especially have begun to push into the 
underlying mesenchyme, and may be described as the dental 
ledges or dental laminae. 
At 16 days the ingrowths have continued to increase as broad 
masses of cells, pushing deeper into the underlying mesenchyme, 
and in the lower jaws the enamel organs may be distinguished 
as expanded structures, each connected by a slightly narrower 
mass of cells with the oral epithelium. In the upper jaws the 
differentiation of the enamel-organs from the remainder of the 
epithelial ingrowth is not so marked. 
At 17 days (fig. 10) the dental papillae are beginning, and the 
enamel-organs in both upper and lower jaws have a crescentic 
outline. In the enamel-organs there is already an indication of 
the differentiation into three layers. As seen in sagittal sec- 
tions, the papillae develop on the posterior side of the enamel- 
organs, thus foreshadowing the axis of growth of the tooth- 
forming organs in the antero-posterior direction. 
Eighteen-day fetus 
Series of frontal sections of 18-day lower jaws, show that the 
enamel-organs are growing over the dental papillae more rapidly 
on the labial and lingual surfaces than elsewhere, and extend 
more posteriorly on these surfaces. There are thus two pro- 
jections of the posterior margin of each enamel-organ as already 
noted by Meyerheim (’98). The labial process is broad and 
thin and extends more posteriorly than the lingual process, which 
is somewhat narrower and thicker. One may here remark, 
therefore, an early difference between the labial and lingual 
part of the enamel-organ. Other differences which will soon 
appear have not yet developed. Thus, the inner layer of the 
enamel-organ is made up of columnar elements which are still 
similar in all parts, both labially and lingually. In the dental 
papilla no columnar odontoblasts are yet seen. 
The enamel-organ remains connected with the surface epithe- 
lium by a broad band of epithelial cells. In the lower jaw, 
