CHAPTER II. 



EVOLUTION OF THE TEETH. 



The teeth are not a part of the skeleton and never, like 

 other juxtaposited hard tissues, unite to each other or to the 

 bones that surround them. They originate from the mucous 

 membranes of the mouth and develop independently of the 

 bones. They are appendages of the buccal mucosa precisely 

 as horn, hair and hoof are appendages of the skin. 



The evolution (growth) of the teeth and their subse- 

 quent retrogression (w^ear) occurs in such distinct succes- 

 sive stages from embryo to old age that the age of animals 

 can be accurately determined at any period of life. The 

 stages of evolution and the stages of retrogression, hov^^ever, 

 vary with each species — the horse, the dog, the ox and the 

 cat each showing individual characteristics. The stages of 

 evolution are distinct in all species, while those of retro- 

 gression are only apparent in the horse, the ox, the sheep 

 and the pig. The dog, the cat and man show no visible 

 distinct stages of wear, owing to the dififerent anatomical 

 construction of the teeth. 



Dental evolution naturally divides itself into three 

 periods : 



1st. The embryonic evolution. 



2nd. Evolution of the temporary teeth. 



3rd. Evolution of the permanent teeth. 



EMBRYONIC EVOLUTION. 



In the embryo of the domestic mammal tooth formation 

 begins at the seventh week of foetal life. The dental ap- 



