66 W. H. F. ADDISON AND J. L. APPLETON, JR. 
jaws distinct differences may be made out between the oral and 
labial sides of the tooth-forming organs. These differences are: 
(1) The enamel-organ is longer labially than on the oral 
side. 
(2) The staining of the inner layer of the enamel-organ 
on the labial side is more intense, and here the cells are slightly 
longer than in other parts of the enamel-organ, measuring 24, 
in length and assuming the typical appearance of ameloblasts. 
Measurements show the similar cells on the lingual side to be 
about 20u in length. It is also to be noted at this age that the 
site of the most advanced cells which are differentiating to 
become ameloblasts is not at the apex of the enamel-organ, as 
is the case in the development of rooted teeth. For as one 
follows the cells of the labial side of the enamel-organ from the 
apex towards the base, while at the apex the cells are columnar 
they become longer as one goes posteriorly, and then towards 
the base of the enamel-organ diminish again. So that the site 
of most advanced differentiation here is a short distance pos- 
terior to the apex on the labial side. This is true also of the 
developing odontoblasts which are longest opposite the tallest 
ameloblasts. 
(3) The outer layer of the enamel-organ on the labial side is 
becoming slightly wavy in outline, and this denotes the begin- 
ning of the papillae, which form such a characteristic part of 
the mature functional enamel-organ (fig. 26). 
(4) The odontoblasts are seen only on the labial side of the 
dental papilla. 
Mitoses are abundant in all parts of the developing tissues. 
T'wenty-one-day fetus 
Thus the anlage of the rodent incisor begins in the usual 
way, and for a short time continues along the typical mammalian 
course. From 19 days onward, however, the differences which 
have already begun, become more distinct and definite. At 21 
days (end of gestation) the enamel-organ has become more defi- 
nitely differentiated into a labial and a lingual region. Of the 
three constituent layers, the inner especially is strikingly different 
