THE ORGANS OF THE INNER GERM-LAYER. 



307 



skin, and the tip of the tooth finally breaks through the epidermal 

 covering. The tooth then acquires a still firmer attachment in the 

 dermis from the fact that, at the surface where the lower margin of 

 the dentine occurs, salts of lime are deposited in the superficial layers 

 of the connective tissue (lh 2 ), and thus a kind of connective-tissue 

 bone, the cementum of the tooth, is produced. 



The finished tooth therefore is constructed out of three calcified 

 tissues, which arise from three separate fundaments. The dentine 



-Kh 



6 HI 

 'zp 



rig. 170. Longitudinal section through an older fundament of a dermal tooth of a Selachian 

 embryo. 



e, Epidermis ; e 1 , the deepest layer of epidermal cells, which are cubical ; sch, mucous cells ; 

 lh 1 , the part of the dermis which is composed of connective-tissxie lamellae ; US, superficial 

 layer of the dermis ; zp, dental papilla ; o, odontoblasts ; zb, dentine ; s, enamel ; sm, enamel- 

 membrane. 



takes its origin from the odontoblast-layer of the dental papilla (mesen- 

 ehyme), the enamel from the epithelial enamel-membrane (outer germ- 

 layer), and the cementum from connective tissue in the vicinity by 

 means of direct ossification. The finished tooth has, moreover, 

 within it a cavity, which is filled with a vascular connective tissue 

 (pulp), the remnant of the papilla. When the enamel-membrane 

 has fulfilled its office it perishes, for in the process of secretion its 

 cells become shorter and shorter, and are finally reduced to flat scales, 

 which are afterwards thrown off. 



In Selachians the formation of the teeth which occupy the edges 

 of the jaws and serve for the comminution of the food differs from 

 this simple process in one important point ; they take their origin, 

 not on the free surface of the mucous membrane, but in its 

 depths (fig. 171). The epithelial tract of the oral mucous membrane 



