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-dermoid, and not 



<4^£r 





Fig. 341. 



X, A section of a cusp 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 filled up with 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 pulp and 

 the merribrana preformativa. 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 the base, Nasmyth's membrane is continued over 

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

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

 c, enamel ; d, Nasmyth 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 



