TISSUES OF THE BODY. 



267 



germ, while its convex external aspect is covered by a similar coating of 

 smaller cells (c). 



But though all is so far tolerably clear, we are now met by the difficult 

 question, so variously answered at different times, as to how these several 

 structures take their origin. 



.Recent investigations, and the researches of Tiersch and KoettiJier, 

 supported at a later date by those of Waldeyer (with which my own subse- 

 quent observations correspond), seem to point to the following conclusions. 



The parts which are contained in the dental sac are of various origin. 

 The dental germ corresponds to a papilla of the mucous membrane, which 

 becomes enclosed in the 

 parietal portion of the fol- 

 licle as with a sheath of the 

 latter. Both these struc- 

 tures have their origin from 

 the proper tissue of the 

 mucous membrane of the 

 foetal jaw. 



The enamel organ, on the 

 other hand, is a produc- 

 tion, by reduplication, of 

 the mucous epithelium, 

 which covers the dental 

 germ, just as a papilla of 

 the mucous membrane is 

 covered by cuticular tissue. 

 But the mass which has 

 grown down into the gum 

 has (in the phase in which 

 we see it in fig. 256) be- 

 come completely separated, 

 by the closing in above, 

 from its original source 

 of origin. 



In order really to under- 

 stand these relations, we 

 must look back to a much 

 earlier period of foetal ex- 

 istence. 



Originally, before any 

 trace of either dental germ 

 or tooth sac is to be seen, 

 the edges of the jaws, 

 which are marked with a 

 slight groove known as 

 the " dental groove," are 

 covered by a thick ridge of epithelium, just over the spots where the 

 future structures are to be formed. This latter has been named the 

 " dental ridge" (2) by Koelliker (see fig. 257, 1 a, 2 a). 



The epithelium soon after commences to grow down from the dental 

 groove into the substance of the mucous membrane, in the form of a 

 leaf-shaped process, which becomes curved downwards and inwards, 

 appearing sickle- shaped, in vertical transverse sections. To this the 



7 



Fig. 257. Developmentof the teeth from Thiersch's preparations 

 of embryonic pigs (vertical transverse sections of the upper 

 jaw). 1, 2. From a small embryo : the right and left halves of 

 the maxilla, a, dental ridge ; 6, younger layer of epithelium ; 

 c, the deepest ; d, enamel germ ; *, enamel organ ; /, dental 

 germ ; g, inner, and A, outer layer of the growing tooth sac. 

 3. From an older embryo : d, the style of the enamel organ ; 

 f, blood-vessel severed; , bony substance. The remaining 

 letters as in 1 and 2. 



