CHAPTER XV, 



STRUCTURE AND DEVELOPMENT OF THE TEETH. 

 BY W. WALDEYER, 



HARDENED structures of the animal organism, similar to those 

 which are called teeth, though certainly presenting very vari- 

 ous histological structure, are found widely distributed both 

 amongst the vertebrate and the invertebrate series. 



o 



With the exception of the larval form of Petromyzon 

 (Ammoccetes); of Amphioxus, Accipenser, and the Lophobranchii 

 (Cuvier), amongst Fishes; of some Toads (Pipa), amongst 

 Amphibia; of the Chelonia, amongst Reptiles ; of the entire 

 class of Birds ; and of the Myrmecophaga, Manis, and Echidna, 

 amongst the Mammals, all vertebrate animals possess teeth. 

 In the whale-bone Whale they are present in the foetal state. 



The anatomical model of a tooth of a vertebrate animal is a 

 large papilla of the mouth or of the pharyngeal mucous 

 membrane, which, in consequence of chemical and histological 

 conversion of its constituents, has acquired a remarkable degree 

 of hardness. And, according to whether the connective tissue 

 substance of the papilla participates in the hardening or not, 

 two large groups of teeth are distinguished dentinal teeth and 

 horny teeth. 



The horny teeth are by far the most simple in their struc- 

 ture. They appear as more or less developed papillae covered 

 with a thick horny investment. They are never continuous 

 with portions of the skeleton, but constitute the transition 

 to other horny formations, as hairs, stings, etc. True horny 

 teeth are met with in the Petromyzidse, the Myxinoids, and in 

 Ornithorhyncus. The whalebone of many whales, and the 

 horny masticating plates of Rhytina Stelleri, though remark- 



