. Chordate morphology. Morphology (Animals); Chordata. germinal layer of epidermis Figure 8-63. Horny tooth of mouth margin of 20-mm larval frog. (After Peyer, 1937) The brachydont type, such as in man, has a limited period of growth during which the tooth is completely formed and the root or roots close. A second type, the hypsodont or high-crowned tooth, is observed in the molars of the horse. This type has a long growth period and both root and crovm are covered by cement, a bony material. The root does not close till late in life. The last type is seen in the incisors of rodents or rabbits
. Chordate morphology. Morphology (Animals); Chordata. germinal layer of epidermis Figure 8-63. Horny tooth of mouth margin of 20-mm larval frog. (After Peyer, 1937) The brachydont type, such as in man, has a limited period of growth during which the tooth is completely formed and the root or roots close. A second type, the hypsodont or high-crowned tooth, is observed in the molars of the horse. This type has a long growth period and both root and crovm are covered by cement, a bony material. The root does not close till late in life. The last type is seen in the incisors of rodents or rabbits and in many other mammals; growth continues through life, and the root remains open. There are several theories as to the origin of mammalian teeth. The simplest is that the several tooth types gradually evolved from the ancestral conical tooth. According to the dimere (du meaning two; meros, parts) theory of Bolk, the primitive reptilian tooth, as well as that of some primitive mammals, the triconodonts, had three cusps (see the front teeth of Tupinambis). Two such teeth fused together would produce a typical molariform tooth. One such tooth minus front and back cusps would form a canine tooth. According to this view, some teeth are trimerous or even polymerous, such as the molars of elephants. Another theory, supported germinal layer of epidermis .enamel dentine B. 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 Jollie, Malcolm. New York, Reinhold
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