64 W. H. F. ADDISON AND J. L. APPLETON, JR. 
immediately laterad to the line of junction of this stalk of the 
enamel-organ to the surface epithelium, the lip furrow is a de- 
pression, the plane of which is continued into the underlying 
mesenchyme by an ingrowth of surface epithelium several layers 
of cells in thickness. It is by the subsequent splitting of this 
epithelial layer into two, that the separation of the lip will be 
effected. 
Nineteen-day fetus 
At 19 days, the enamel-organ in the upper jaw (fig. 11) is 
erescentic in outline in sagittal section, and in the lower Jaw 
(fig. 12) is more elongated and conical in shape. At this age 
odontoblasts are first seen as columnar cells on the labial aspect 
of the mesodermal papilla. Three layers are recognizable in 
the enamel-organ, but the middle layer (enamel pulp), as has 
been also described by Sachse (94) for the mouse, is extremely 
thin, and therefore is not present in the great quantity typically 
seen in the development of rooted teeth. It appears as a more 
lightly stained zone between the inner and outer layers, and is 
thickest at the basal end of the enamel-organ as shown in figures 
11 and 12. It averages about 20u in thickness and is made up 
of stellate cells loosely arranged. Already there is an indica- 
tion of a compact arrangement of the two or three rows of cells 
next the inner layer of the enamel-organ, which will result in 
the so-called stratum intermedium seen at later ages. This 
middle layer is also slightly more abundant at the anterior 
end in the region where the enamel-organ is continuous with the 
stalk which joins it to the oral epithelium. 
At this age the enamel-organs in the lower Jaws have a greater 
total length than those in the upper, and especially in the lower 
Fig. 10 Longitudinal section of upper jaw of 17-day fetus, showing tooth 
anlage of incisor, with the enamel-organ longer labially than lingually when 
measured from the point of junction of the stalk of the enamel-organ. The 
dental papilla is on the posterior aspect of the enamel-organ. % 70. 
Fig. 11 Longitudinal section of upper incisor anlage of 19-day fetus of al- 
bino rat, showing the crescentic outline of the enamel-organ, its greater length 
labially than lingually, and its thickened basal margin. X 70. 
Fig. 12 Longitudinal section of lower incisor anlage of 19-day fetus of albino 
rat, showing the conical outline of the enamel-organ and its greater length than 
in the upper jaw at the same age. X 70. 
