STRUCTURE AND GROWTH OF INCISOR TEETH AT 
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Fig. 1 Cranium of a 5-month albino rat. X 2. 
Fig. 2 Cranium of a 5-month albino rat, with the bony alveoli dissected 
away to show the entire length of the incisor teeth. X 2. 
The incisors are permanently-growing (or rootless) teeth, 
while the molars have a definite limited period of development 
and acquire roots. A wide diastema separates the incisors 
from the molars as may be seen by reference to figure 1. The 
incisors are strongly curved and Owen (’40-’45) has described 
the lower incisor as being the smaller segment of a larger circle, 
and the upper incisor as the larger segment of a smaller circle. 
In the lower incisor of the albino rat this statement needs a 
slight modification. For while the curvature of the upper in- 
