v THE TEETH ■ 325 



the rest of the ectoderm so as to remain connected with it only by a 

 narrow stalk or isthmus. 



The tooth is built up of precisely the same elements as the 

 placoid scale — dentine, enamel and basal plate. Its modifications are 

 such as to make it more efficient for its special purpose. The 

 projecting spine becomes exaggerated to form the functional part of 

 the tooth : it remains conical, or it becomes a flattened blade with 

 plain or serrated edge, or it becomes a low flattened crushing plate. 



To secure greater strength the pulp may become traversed by 

 hard trabeculae (vaso-dentine). 



Eegarding each of the three elements mentioned above there is a 

 certain amount of controversy. As regards the dentine there is the 

 question of its origin — whether it is to be regarded as calcified 

 matrix or as modified cytoplasm. The evidence of Lepidosiren — 

 which on account of the size of its cell elements is always of weight 

 in such questions — seems very clearly on the side of the latter view. 

 As shown in Fig. 161, B, the cytoplasm of the odontoblast passes 

 uninterruptedly into the calcified dentine, the spaces between the 

 odontoblasts on the other hand dying away as the dentine is 

 approached. But if in a relatively archaic creature like Lepidosiren 

 the main part of the dentine is undoubtedly modified cytoplasm this 

 at once raises a strong presumption in favour of the same being the 

 case in the higher Vertebrates even if it be not actually obvious. 



Again as regards the enamel it is taught practically universally 

 that it is formed after the manner of an internal cuticle by the 

 cells of the enamel epithelium. This idea has come down to us 

 from the days of the early investigators who devoted themselves 

 especially to the investigation of Man and those Vertebrates most 

 closely allied to him. In those days the structure of the lower 

 animals was interpreted according to the data obtained from Man 

 and his allies. The whole outlook was the opposite of that which 

 holds in these evolutionary days when the accepted principle of all 

 morphological work is to interpret the higher and more complex 

 animals by data obtained from those lower in the evolutionary scale. 

 Applying this principle to the case of the teeth of the most archaic 

 Vertebrates we see in the Elasmobranch fishes that the outermost 

 layers of the dentine develop the special modifications already 

 alluded to — extreme denseness and hardness, transparency and high 

 refraction, reduction of the proportion of organic material, reduction 

 of the tubular cavities. Here the enamel is undoubtedly modified 

 dentine. 



But if this be so there are only two alternatives open to us in 

 interpreting the enamel of the higher Vertebrates. It is either to 

 be regarded as a further stage in the differentiation of the outer 

 layer of dentine or it is to be regarded as something quite new, a 

 new substance formed by the enamel epithelium. This latter is 

 the generally accepted view and in accordance with it. the hard 

 layer on. the teeth of fishes was given by Williamson the name 



