DEVELOPMENT OF THE DIGESTIVE TUBE 605 



In the human and pig embryos, the anterior portion or body of the tongue 

 arises through the fusion of two ventro-medial swellings of the mandibular 



arches (fig. 285B). The root of the tongue takes its origin from areas of ele- 

 vated tissue upon the ventral ends of the hyoid arches and in the adjacent 

 area between the hyoid and first branchial visceral arches (fig. 285B). This 

 elevated tissue is known as the copula. A small, insignificant area, the tuber- 

 culum impar, emerges from the medio-ventral area between the mandibular 

 and hyoid visceral arches (fig. 285B). Stages in tongue development in the 

 human embryo are shown in figure 285A-E. 



4) Teeth: a) General Characteristics. Teeth are of two types: 



( 1 ) horny teeth and 



(2) bony or true teeth. 



Horny teeth are found in cyclostomatous fishes, the larval stages of frogs 

 and toads, and in the prototherian mammal, Ornithorhynchus. 



Most vertebrates possess true or bony teeth, although they are absent in 

 some fishes (e.g., the sturgeon, pipefishes, and sea horses), turtles, and birds. 

 Among the mammals, certain whales lack teeth, and, m Ornithorhynchus, 

 vestigial bony teeth are formed before hatching, to be lost and supplanted by 

 cornified epidermal teeth. Teeth are lacking also in the edentates, Myrme- 

 cophaga and Manis. 



True or bone-like teeth have essentially the same general structure in all 

 vertebrates. A tooth possesses three general areas (fig. 286E): 



( 1 ) crown, 



(2) neck, and 



(3) root. 



The crown projects from the surface of epithelium overlying the jaw or 

 oral cavity, while the root is attached to the jaw tissue. The neck is the re- 

 stricted area lying between the root and the crown. 



Teeth generally are composed of two substances, enamel and dentine. Some 

 teeth, however, lack enamel. Examples of the latter are the teeth of sloths and 

 armadillos. The tusks of elephants also represent greatly modified teeth with- 

 out enamel. Some teeth have the enamel only on the anterior aspect, such as 

 the incisors of rodents. 



Teeth may be attached to the jaw area in various ways. In sharks, the teeth 

 are embedded in the connective tissue overlying the jaws (fig. 287F), whereas 

 in most teleosts, amphibia, reptiles, birds, and mammals, they are connected 

 to the jaw itself (fig. 287A-D). In many vertebrates, such as crocodilians and 

 mammals, the tooth is implanted in a socket or alveolus within the jaw tissue 

 (fig. 287C, D). In other forms, the tooth is fused (i.e., ankylosed) to the 

 upper surfaces of the jaw (fig. 287A, B). A tooth inserted within a socket 

 or alveolus of the jaw is spoken of as a thecodont tooth, while those teeth 



