52 DEVELOPMENT OF TEETH CHAP. 
and other vertebrates aS was at one time assumed. But in 
order to explain this very important part of the subject it will 
be necessary to give some account of the development of the 
teeth. The type selected is the Hedgehog, which has been 
recently and carefully described by Dr. Leche of Stockholm, 
Fic. 37.—Two stages in the development of the teeth of a Mammal (diagrammatic sec- 
tious). alv, Bone of alveolus ; dent, dentine; dent.s, dental sac ; en, enamel ; 
en.m, enamel membrane ; en.m7, enamel membrane of permanent tooth ; en.plp, 
enamel pulp ; gr, dental groove ; Jum, dental Jamina ; lam’, part of dental lamina 
which grows downwards below the tooth germ; 2, neck connecting germs of milk 
and permanent tooth ; pap, dental papilla ; pap", dental papilla of permanent tooth. 
(After O. Hertwig.) 
which type las 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 ? (Schmelzorgane, enamel organs) 
are developed here and there as thickenings in the form of buds 
1 Morph. Jahrb. xix. 1892, p. 502. 
