THE DEVELOPMENT OF THE TEETH 167 



milk or decidual teeth vary with race, climate and nutritive conditions. Usually 

 the teeth are cut in the following sequence : 



DECIDUAL OR MILK TEETH 



Median Incisors sixth to eighth month. 



Lateral Incisors eighth to twelfth month. 



First Molars twelfth to sixteenth month. 



Canines seventeenth to twentieth month. 



Second Molars twentieth to twenty-fourth month. 



The permanent teeth are all present at the fifth year. They are located 

 mesial to the decidual teeth (Fig. 160), and, before the permanent teeth begin 

 to erupt, the roots of the milk teeth undergo absorption, their dental pulp dies 

 and they are eventually shed. The permanent teeth are "cut" as follows: 

 (McMurrich in Keibel and Mall, vol. 2, p. 354). 



First Molars seventh year 



Median Incisors eighth year. 



Lateral Incisors ninth year. 



First Premolars tenth year. 



Second Premolars eleventh year. 



> . . . thirteenth to fourteenth year. 



Second Molars J 



Third Molars seventeenth to fortieth year. 



Dental anomalies are frequent and may consist in the congenital absence of some or all 

 of the teeth, or in the production of more than the normal number. Defective teeth are fre- 

 quently associated with hare lip. Cases have been noted in which, owing to defect of the 

 enamel organ, the enamel was entirely wanting. Many cases in which a third dentition occurred 

 have been recorded and occasionally fourth molars may be developed behind the wisdom teeth. 



The teeth of vertebrates are homologues of the placoid scales of elasmobranch fishes 

 (sharks). The teeth of the shark resemble enlarged scales, and many generations of teeth are 

 produced in the adult fish. In some mammalian embryos three or even four dentitions are 

 present. The primitive teeth of mammals are of the canine type and, from this conical tooth, 

 the incisors and molars have been differentiated. 



