708 



THE MICROSCOPE. 



situated by the side of the old ones. Blandin was the first 

 to point out that the teeth are developed in the mucous 

 membrane, in a similar way to hair and nails. Other ob- 

 servers have been led to the same conclusion ; and, more 

 lately. Professor Goodsir demonstrated that the teeth are 

 first formed in grooves of the mucous membrane, and sub- 

 sequently converted into closed sacs by a process of involu- 

 tion, and that their final adhesion to the jaw is a com- 

 paratively late part of the process. It is now generally 

 conceded that teeth belong to the muco-derrnoid, and not 





%/^9 



r« *^ *^ ^ -~ 7 r-^- - 







^^-^€>^ 



Fig. 341. 



I, A section of a cusj^ of the posterior molar, upper jaw of a Child. The inner 

 outline represents it before the addition of acetic acid — the outer after- 

 wards, when Nasmyth's membrane g is seen raised up in folds ; /, the enamel 

 organ ; e, the dentine. The central portion is tilled up witli pulp. 2. Edge 

 of the pulp of a molar cusp, showing the first rudiment of the. dentine, com- 

 mencing in a perfectly transparent layer between the nuclei of the ]iulx> and 

 the membrana pre/ormativa. 3, Nasmyth's membrane detached from the 

 subjacent enamel by acetic acid. 3, The stellate-cells of the enamel-organ. 

 5, Tooth of the Frog, acted on by dilute hydrochloric acid, so as to dissolve 

 out the enamel and free Nasmyth's membrane. The structure of the dentine 

 e is rendered indistinct. At tlie base, Nasmyth's membrane is continued over 

 the bony substance at z, in which the nuclei of the lacuna} are visible. (After 

 Huxley.) 6. Decalcified tooth-structure; a, the dentine; b, enamel organ; 

 c, enamel ; d, Nasniyth's membrane. 



to the periosteal, series of tissues; that, instead of standing 

 in close relation to the endo-skeleton, they are part of the 

 dermal or exo-skeleton ; their true anologues being the 

 hair, and some other epidermic appendages. Professor 

 Huxley has proved that^ although teeth are developed in 



