Vertehrata. 



339 



in the following way : a papilla of the dermis, or of the corresponding 

 connective tissue of the mucous membrane, grows into the epidermis, 

 or what is the same thing, the epithelium of the mouth. This papilla 

 secretes, on its upper surface, a covering of dentine, a substance 

 as hard as bone, the structure of which will be considered later; whilst 

 the cylindrical cells, constituting the lowest layer of the epithelium, 

 corering the papilla, secrete on their under sides a layer of still harder 

 material, the enamel. Between the papilla and the epithelium 



■y)6 ep" 



Fig. 279. Diagram of various developing teeth. To the left in each figure a very 

 young one is represented, followed to the right by an older or several older teeth in 

 succession. A simplest form, B — C more complicated forms.. Enamel black ; dentine 

 perpendicularly shaded ; connective tissue dotted, h connective tissue, eo enamel organ, 

 ep epithelium, ej>' epithelial papilla (incipient enamel organ), ep" older epithelial papilla, 

 p dermal papilla (pulp of tooth). — Orig. 



a hard cap is thus formed, which consists within of a layer of dentine, 

 secreted by the papilla, and externally of a layer of enamel, secreted 

 by the epithelium ; the two layers are inseparably connected, and 

 together make up the } oung tooth. The dentine is gradually thickened 

 by the secretion of new material by the papilla, which at the same time 

 becomes smaller, and is finally often reduced to a relatively small 

 structure, the pulp, within the tooth. The enamel is increased 

 by the deposition of new material on its surface, it never 

 attains so great a thickness as the dentine, and is often not 

 distributed over the whole tooth, but occurs only at its apex. As a 

 rule, however, development is somewhat more complicated (.see Pig. 

 279 B — 0) ; before the formation of the papilla a thickening of 

 the epithelium. occur.s at a given spot, and in connection with it there 

 is an ingrowth into the connective tissue (ep'), which is usually so deep 

 that the point of the tooth does not project at all into the true layer of 

 epithelium, but lies exclusively in the sunken portion (Fig. 279 G) ; 

 this latter often only retains its connection with the epithelium by a 

 thin cord, and, indeed, is often completely separate d from it. In other 



z 2 



