of the Enamel of Elasmobrancli Fishes. 55 



Each dentine papilla forms upon its surface a specialised layer 

 which is derived from spindle-shaped cells, sending out immensely 

 elongated processes which run nearly parallel with the surface. 

 There is a large amount of intercellular substance formed so that the 

 cell processes ultimately become inconspicuous, and a lamination or 

 fibrillation of this layer only remains ; in this stage it is exceedingly 

 resistant to staining. 



The layer is also permeated by cell processes which run through 

 it at right angles to its surface, and these persist. It is found that 

 this layer is the site of the so-called enamel formation, the first 

 named cell processes giving rise to its lamination, and the last men- 

 tioned cell processes giving rise to the tube system which per- 

 meates it. 



In all mammals dentine calcification commences at the very out- 

 side of the dentine papilla, and nothing at all corresponding to this 

 specialised layer exists at any period. 



But in the Elasmobranch fishes the calcification of the dentine, 

 whether it be an osteodentine as in Lamna or a fine-tubed dentine as 

 in most others, does not take place at the outer surface of the entire 

 dentinal papilla, but along the deeper side of the specialised layer, 

 thus soon cutting it off from any free communication with the body 

 of the pulp. 



This layer, in the extent to which it is developed, bears a ratio to 

 the thickness of the ultimate ** enamel." Over it lie the columnar 

 epithelial cells of the enamel organ, which present several peculiari- 

 ties ; they are found to grow to three or four times their original 

 size quite suddenly ; that is to say, on one tooth germ they are small, 

 on the very next they are enormous, and this is the tooth germ in 

 which the specialised layer has attained to its maximum. Then on 

 the next older germ the ameloblasts have fallen to their original size, 

 and have lost their distinctness of outline. 



The great growth of these cells just at one particular stage of tooth 

 development, their subsequent immediate atrophy, and the fact that 

 their length bears also a direct ratio to the thickness of the enamel 

 formed, renders it impossible to suppose that they can be function- 

 less, and it is suggested that they furnish the lime salts for the 

 calcification of the specialised layer of the dentine papilla before 

 alluded to, there being difficulties in the way of supposing that the 

 dentine papilla does so. 



In that case the disputed enamel layer of the finished tooth does 

 not correspond precisely either to the enamel or to the dentine of the 

 completed mammalian tooth, but is a joint product of the epiblastic 

 enamel organ and the mesoblastic dentine papilla. 



The general conclusion arrived at is that, just as the whole teeth 

 of the Elasmobranchs present the simplest known form of tooth 



