52 



DEVELOPilKNT Ol" • TEE Tl I , 



and other vertebrates as was at one time assumed. But in 

 order to explain this very important pari of ihe subject it will 

 be necessary to give some account of the development of the 

 teeth. The type selected is the Hedgehog, which lias been 

 recently and carefully descrilied by Dr. Lecln^ of Stockholm, 



-x'^r'—^Z'CLrrv 



cn.m^ 



dents 



Fni. o7. — Two stages in the development nC the teeth of a IManiiiial ((liagranunatic sec- 

 tions), alv, Bone of alveolus : <lciit, dentine ; dent.a, dental sac : eii, enamel ; 

 cn.m, enamel membrane ; en.m^, enamel membrane of permanent tooth ; en.plp, 

 enamel pulp ; gr, dental groove ; lam, dental lamina ; law', part of dental lamina 

 which grows downwards lielow the tooth germ ; n, neck connecting germs of milk 

 and permanent tooth : paj), dental papilla ; jnqi-, dental papilla of permanent tooth. 

 (After 0. Hei-twig.) 



which type has furthermore the advantage of being a " central " 

 type of mammal. The first step in the formation of the teeth 

 is a continuous invagination of the epithelium covering the jaw 

 to form a deepish wall of tissue running in the thickness of the 

 jaw ; this is perfectly continuous from end to end of the lower 

 jaw. From this " common enamel germ " {Schmelzleiste of the 

 Germans^) "special enamel germs " (Sc/imehnrqanr, enamel organs) 

 are developed here and there as thickenings in tlie form of buds. 

 1 3fo)pJi. Jahrb. xix. 1892, p. 502. 



