68 W. H. F. ADDISON AND J. L. APPLETON, JR. 
coming odontoblasts. In the basal half of the papilla (fig. 14), 
odontoblasts occur only on the labial side opposite the tall amel- 
oblasts, the peripheral cells of the other sides being still irregu- 
lar or cuboidal in shape. Farther forwards the odontoblasts 
are found also on the lateral and mesial surfaces of the dental 
papilla, but not on the lingual. In the apical one-fourth of the 
dental papilla odontoblasts occur all round, measuring 20 to 
24 in length, and are engaged in the formation of dentine (fig. 
13). The dentine is thickest on the labial side. 
In the region where enamel and dentine formation has begun 
no mitoses were seen in the formative ameloblastic and odonto- 
blastic cells, but posteriorly, where the deposition of enamel 
and dentine has not yet commenced, many mitoses occur in the 
layers of developing ameloblasts and odontoblasts, as well as 
elsewhere. The nearer one approaches the basal margin of the 
enamel-organ the more numerous are the mitoses and it is ap- 
parent that it is principally in this region that growth by addi- 
tion of new cells is taking place. 
One day old 
At the end of the first day of post-natal life, there has been 
great progress in the enamel and dentine formation, and the 
narrow, pointed outline of the tooth has been already laid down. 
In the upper jaw the teeth measure about 2.3 mm. in length and 
in the lower jaw about 3 mm. Definite changes in its relation 
to the oral epithelium have occurred also at the anterior end of 
the tooth. The original epithelial stalk connecting the enamel- 
organ with the oral epithelium has increased in size and the end 
Fig. 13 Cross-section of developing lower incisor of 21-day fetus of albino 
rat, nearer the anterior extremity of the tooth than figure 14. Shows the greater 
thickness of the labial side of the enamel organ, as compared with that of the 
other sides, and shows odontoblasts around the entire periphery of the pulp. 
Enamel and dentine formation has begun. X 110. 
Fig. 14 Cross-section of developing lower incisor of 21-day fetus of albino 
rat, posterior to the region shown in figure 18. No enamel or dentine yet formed 
at this point. Odontoblasts highest on the labial aspect, decreasing in height 
laterally but not yet differentiated as columnar elements on the lingual side. 
Enamel-organ thickest on the labial side. 110. 
