TEETH 



53 



the age of eruption of the third molar (the ''wisdom tooth") 

 to that of the second molar is similar in the rat to that found 

 for these teeth in the permanent dentition of man. 



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 (Addison 

 and Appleton ('15)). The incisors are strongly curved and 

 Owen (1840-1845) 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 (verified by Hammett and 

 Justice '23). 



The lower incisor of a five months animal forms a segment of 

 about four-fifths of a semicircle (140-145°). 



The times of the early stages of development of the incisors 

 were as follows: 



14 day fetus^ — slight thickening of oral epithelium. 



15 day fetus— distinct thickening and growth inwards of oral epithelium. 



16 day fetus— dental ledge and beginning of flask-shaped enamel organ. 



17 day fetus^ — dental papilla with crescentic enamel organ capping it. 

 19 day fetus — both ameloblasts and odontoblasts differentiated. 

 New-born animal — enamel and dentine formation begun. 



8 to 10 days — eruption of the tooth. 



At birth there are blood-vessels in the enamel organ of the 

 molar teeth (Addison and Appleton '22). 



The rate at which the teeth increase in length during their 

 formative period and prior to attrition is given in table 18. 



Average growth of upper incisor 0.52 mm. and of lower incisor 0. 

 per da}\ 



