44 Basic Structure of Vertebrates 



the seventh month after birth and is completed by the end of the sec- 

 ond year. Of the permanent set, the first to erupt are the most anterior 

 molars which appear in the sixth year. The last to erupt are the hind- 

 most molars or "wisdom teeth." 



The shape of a tooth is determined by that of the tooth-germ. If 

 the ameloblast and odontoblast layers become folded, the form of the 

 tooth is correspondingly modified. The complex teeth of ancient 

 "labyrinthodont " amphibians (Fig. 41) and the elaborately ridged 

 grinding teeth of the sheep and horse (Fig. 42E) are thus produced. The 



Fig. 41. Part of a cross section of a 

 tooth of an ancient labyrinthodont am- 

 phibian. The tooth wall is elaborately 

 folded inward and outward in relation to 

 the central pulp cavity. (Courtesy, Owen: 

 "Comparative Anatomy and Physiology 

 of Vertebrates," London, Longmans, Green 

 & Co., Ltd.) 



multiplication of roots, as in large molar teeth (Fig. 43), results from 

 the budding of the odontoblast layer of the dental papilla. 



Comparative Morphology of Teeth 



In the structure of teeth there are many variations. The essential 

 part of a tooth is the dentine. Teeth of many vertebrates lack enamel 

 (e.g., some fishes, and, among mammals, the sloth and armadillo). The 

 elephant's tusk has no enamel, except that the tip is covered by a small 

 cap of enamel which soon wears off. But there is never enamel without 

 dentine. 



A definite layer of cement on the root does not ordinarily occur in 

 vertebrates other than mammals unless it be regarded as represented 

 by the bony tissue which literally "cements'* the ankylosed tooth to 



