84 MICROSCOPIC ANATOMY OF THE TEETH 



than elsewhere, the probable reason for which will be 

 considered in treating of the development of enamel. 



Tubular Enamel 



Tubular enamel may be described as an enamel which 

 is normally penetrated by tubes either from the dentine 

 or from its outer surface. 



The penetration of the enamel by tubes from the dentine 

 is seen in many fishes, in marsupials in its most complete 

 form, and also in the Hyracoidea (in Hyrax), among Rodentia 

 in the Jerboa (Dipus), and in the Insectivora in the Shrews 

 (Sarex). 

 Tubular Penetration by tubes from the outer surface of the enamel 



toFiS? is often found in fish- 

 In the Plagiostomi it is seen in Cestracion, Lamna, and in 



many sharks both recent and fossil, and among osseous 

 fish, in the Gadidse, Labridae, and Sparidse or sea-breams, 

 being especially well marked in the genus Sargina, and the 

 enamel of a member of this group, the Sargus ovis or Sheep's 

 Head fish, has long been taken as the typical example of 

 this penetration from the outer surface of the enamel. 

 The outer layer of the teeth of Plagiostome fish differs in some 

 respects from the enamel of higher forms, for besides the 

 penetration by tubes it shows a marked transverse striation 

 and contains lacuna-like spaces, and in some examples, 

 as Lamna, there is a differentiated outer layer. From 

 a study of the mode of development of this structure in 

 Elasmobranch fishes, C. S. Tomes (18 d) concludes 

 that the outer hard layer in the teeth of Selachia and 

 Ganoid fish corresponds neither to the dentine nor the 

 enamel of Mammalia. Having no collagen matrix it cannot 

 be looked upon as dentine, 'while', he says, 'its organic 

 matrix is beyond question furnished by the mesoblastic 

 dentine papilla, the epiblastic ameloblasts over it are in 

 a state of development which implies that they take an 

 active part, and that the tissue is a joint production '. 

 He considers that the part played by the ameloblasts is 

 probably the elaboration of the calcifying salts in this 

 enamel-like substance. In his classification of enamels, 



