ON THE DEVELOPMENT OF THE TEETH 



and I think we must be content for the present to consider its 

 existence and its structure as ultimate facts, not expHcable by the 

 Cell Theory. It is particularly worthy of notice that in the Skate 

 the dermal teeth or plates on the upper surface of the head have as 

 distinct a layer of enamel as those of the mouth, though in this case 

 there is most assuredly neither rudimentary capsule nor " enamel 

 organ." 



In a morphological point of view, the relations of the cement show 

 it to be homologous with the enamel. In a very beautiful section 

 of a human tooth from Mr. Busk's cabinet, the upper portion of the 

 cement exhibits in places a very distinct transverse striation, resem- 

 bling its perfect enamel. But the transition of the one structure into 

 the other is best exhibited in the young Calf by the cement of the 

 fang of a molar which had not cut the gum. Here it is a white 

 substance, from which generally a fitting section can be cut only with 

 some difficulty, in consequence of its friability. The layer is about 

 I -40th of an inch thick, and consists of an external delicate structure- 

 less Nasmyth's membrane ; internal to which three-fourths of the 

 thickness of the layer are formed by parallel fibres i -5000th of an 

 inch in diameter, quite structureless, and completely resembling 

 enamel fibres, but absolutely enormous (as much as i-6oth of an inch) 

 in length. These fibres were softened and rendered pale by the 

 action of caustic ammonia. The inner fourth of the layer of cement 

 was composed of an inextricably interlaced body of such fibres, united 

 into a mass, which in some places was almost homogeneous, by 

 calcareous salts, and containing here and there lacuna:; i-i6ooth of an 

 inch in length, similar to those of bone. That this structure was the 

 young cement is certain, inasmuch as no enamel is formed on the 

 fang of the tooth, to say nothing of the presence of the lacunae. On 

 the root of the fang of the molar in front of this, which had cut the 

 gum some time, and had come into use, the cement had the ordinary 

 structure. It may be worth while to add that in these teeth the 

 capsule, though closely connected with the outer surface of the fang, 

 could be readily stripped from it, and then exhibited a layer of 

 epithelium upon its inner surface, showing clearly that the cement was 

 not derived from its ossification. 



It may be concluded, then, — 



1. The teeth are true dermic structures, formed by the deposit of 

 calcareous matter beneath the basement membrane of a dermic 

 papilla, or that which corresponds with one. 



2. Neither the capsule nor the " enamel-organ," which consists of 

 the epithelium of both the papilla and the capsule, contribute directly 



