STRUCTURE AND GROWTH OF INCISOR TEETH Of 
nasal chamber, while the right upper (fig. 28) has recurved and 
grown into the maxilla. 
In The Wistar Institute rat colony, at the time when the 
animals were fed on bread and milk, frequent examples of this 
and similar conditions were found, but now under a more varied 
mixed diet they practically never occur. 
Beretta (13) has recently made an analysis of these abnor- 
malities and has classified them in three groups. 
(1) Overgrowth of the upper and lower incisors through 
lack of an opposing tooth. 
(2) Overgrowth of the incisors of the upper and lower jaws 
through deviation of the jaws. , 
(3) Prognathism of the lower jaw, and as a result, over- 
growth of the incisor of the lower Jaw. 
In the present instance, diet seemed to be the controlling 
factor, probably by reason of its influence on the hardness of 
the bone of the alveoli from which the teeth grew out. 
SUMMARY 
The rate of growth of the upper and lower incisor teeth of 
Mus norvegicus albinus, in the mature animal, averages 2.2 
and 2.8 mm. per week, or 12.5 cm. and 14.5 cm. per year, 
respectively. 
Growth is due primarily to the proliferation and growth 
of cells at the basal end of the enamel-organ, where new enamel- 
forming cells arise, and at the basal end of the dental papilla 
where new dentine-forming cells develop. 
The enamel-organ of the adult forms a narrow circular band 
around the basal end of the tooth, and extends forward from 
this on the labial side only. It coincides in its lateral bound- 
aries with the enamel, and extends along the entire imbedded 
portion of the tooth. Anteriorly, it comes in contact with the 
epithelium of the gingival margin, and is carried out continually 
as a narrow band of cells lying on the enamel, between the latter 
and the gingival epithelial tissue. 
The first indication of the anlage of the incisors appears in 
14-day-old fetuses. In fetuses, 21 days of age (just before 
JOURNAL OF MORPHOLOGY, VOL. 26, NO. 1 
