92 W. H. F. ADDISON AND J. L. APPLETON, JR. 
birth), enamel and dentine formation is beginning. In animals 
1 day old the upper and lower teeth measure 2.3 and 3 mm. At 
8 to 10 days the teeth erupt, and at 10 days measure 7 and 11 
mm. respectively. This period is therefore characterized by 
the rapid elongation of the teeth. 
The process of attrition begins within a few days after erup- 
tion, so that by 19 or 21 days of age, the typical occlusal surface 
is formed. Up to the time of eruption the anterior end or apex 
of the tooth is immediately under the oral epithelium, while 
the basal or growing end is continually progressing posteriorly. 
After eruption, the basal end becomes nearly stationary in 
position, while the whole tooth structure is continually moving 
forward. The extra-gingival length of the tooth is kept con- 
stant, however, by the attrition of the occlusal surface, either 
through use in gnawing or by the action of the opposing teeth. 
The histogenesis of the enamel-organ is practically com- 
pleted by the 4th day after birth, although it does not attain its 
final relations to the tooth as a whole, until after eruption. 
In the 18-day fetus the enamel-organ is similar in all parts, and 
the cells of the inner layer measure the same, both lingually and 
labially. From this period forwards, however, the labial por- 
tion continues to progress towards its fully differentiated func- 
tional structure, while the lingual portion retrogresses, until 
at 4 days after birth the latter is disrupted, by the ingrowth 
of the surrounding connective tissue. Contrasting the cells 
of the inner layer—the potential ameloblasts—on the labial and 
lingual sides, they are practically the same in the 18-day fetus, 
but at 19 days they are found to measure 24 and 20 uwrespectively. 
In the 21-day fetus, they measure 30 to 34 and 12 u, and 1 day 
after birth the true ameloblasts on the labial side have increased 
to 40 uw, while the non-functional cells of the lingual side are 
only 10 u in height. At 4 days, the latter cease to form a con- 
tinuous layer, by reason of the dispersion of the cells by the 
surrounding connective tissue, except at the basal formative 
region. 
Characteristic of the permanently-growing enamel-organ are 
the epithelial papillae, formed by the elevations of the outer 
