The Teeth 



311 



ward beneath the oesophagus and above the heart and the distal 

 ends of the two lobes become enlarged and commence dividing 

 into small lobes. The lobes elongate greatly and give rise to 

 buds which end in somewhat enlarged ampullae, which continue 

 to subdivide in this manner, the terminal infundibuli constitut- 

 ing the air cells, while the tubes leading from them are the 

 bronchioles, the larger ones are the bronchii, the original tube 

 leading from the floor of the pharynx becomes the trachea, and 

 the slit which first appeared in the bottom of the pharynx forms 

 the glottis. 



The Teeth. 



The teeth appear very early in the life of the embryo and orig- 

 inate from the epiblast of the stomatodeum in the form of a lon- 

 gitudinal invagination of thickened epithelium along the border 

 of the jaw, which eventually sinks down into the substance of 

 the jaw as a continuous ridge, known as the common enamel 

 germ. Later, the ridges become enlarged at intervals, to consti- 

 tute the individual enamel organs, while the portions between 



Fig. 43. Tl.: -ive sta,tcesin the development of a tooth- 



germ of a pig embryo ( after Frey and Thiersch): a, b, c, layers 

 of thickened oral epithelium, showing dental groove on sur- 

 face in 3 ; e, enamel organ ; f, dental papilla ; g, h, internal and 

 external layers of follicle wall ; i, blood-vessel ; k, maxilla; d, 

 epithelial ingrowth, the end of which expands into the enamel 

 sac. (Heisler). 



