XIII 



PHYLUM CHORDATA 



541 



Primates by the absence of tin's power of opposition and by the 

 relative length of the tarsus, which exceeds that of the metatarsus. 

 Digestive Organs. Teeth are present in nearly all Mammals, 

 but in some they are wanting in the adult condition (Whalebone 

 Whales and Platypus). In 

 Echidna teeth are not pre- 

 sent even in the young. In 

 some of the Ant-eaters teeth 

 are developed in the foetus 

 and are thrown off in utero 

 the adult animal being 

 devoid of them. 



Teeth, as already ex- 

 plained in the general ac- 

 count of the Craniata 

 (p. 84), are developed partly 

 from the epidermis and 

 partly from the underlying 

 dermis. In the Mammals 

 each tooth is lodged in a 

 socket or alveolus in the 

 jaw. The part of the tooth 

 developed from the epider- 

 mis is 'the enamel ; the 

 remainder of the tooth 

 dentine, cement and pulp 

 being formed from the sub- 

 jacent mesodermal tissue. 



Along the oral surface of 

 the jaw is formed a ridge- 

 like ingrowth of the ecto- 

 derm the dental lamina 

 (Fig. 1194, lam.). The posi- 

 tion of this is indicated 

 externally by a groove the 

 dental groove (gr.), and from 

 it a bud is given off in 

 the position to be occupied 

 by each of the teeth. This 



FIG. 1193. Diagrammatic sections of various forma 

 of teeth. I, incisor or tusk of Elephant with pulp- 

 cavity persistently open at base ; II, human 

 incisor during development, with root imperfectly 

 formed, and pulp-cavity widely open at base ; ///, 

 completely formed human incisor, with pulp- 

 cavity opening by a contracted aperture at base 

 of root ; IV, human molar with broad crown and 

 two roots ; V, molar of the Ox, with the enamel 

 covering the crown deeply folded, and the de- 



pressions filled up with cement ; the surface is 

 worn by use, otherwise the enamel coating would 

 be continuous at the top of the ridges. In all the 

 figures the enamel is black, the pulp white, the 

 dentine represented by horizontal lines, and the 

 cement by dots. (After Flower and Lydekker.) 



becomes constricted off as 

 a conical cap of cells the 

 enamel-organ which re- 

 mains in continuity with the 

 dental ridge only by a narrow isthmus. This cap-like form is 

 brought about by the development of a papilla of condensed dermal 

 tissue the dental papilla (pap,), which pushes upwards against 

 the enamel-organ. On the surface of this papilla, in contact with 



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