. The chordates. Chordata. 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 ; 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 te
. The chordates. Chordata. 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 ; 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 (, 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : Blakiston
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