20 GENERAL ANATOMICAL CHARACTERS 



by others. These animals are called Monophyodont. But in the 

 larger number of mammals, certain of the teeth are preceded by 

 others, which may be only of a very transient, rudimentary, and 

 functionless character (being in the Seals, for example, shed either 

 before or within a few days after birth), or may be considerably 

 developed, and functionally occupy the place of the permanent teeth 

 for a somewhat lengthened period, during the growth and develop- 

 ment of the latter and of the jaws. In all cases these teeth 

 disappear (by the absorption of their roots and shedding of the 

 crowns) before the frame of the animal has acquired complete 

 maturity, as evidenced by the coalescence of the epiphyses of the 

 osseous system. As these teeth are, as a general rule, present 

 during the period in which the animal is nourished by the milk of 

 the mother, the name of "milk-teeth" (French dents de hit, 

 German milchzahne) has been commonly accorded to them, although 

 it must be understood that the epoch of their presence is by no 

 means necessarily synchronous with that of lactation. Animals 

 possessing such teeth are called Diphyodont. No mammal is known 

 to have more than two sets of teeth ; and the definite and orderly 

 replacement of certain members of the series is a process of quite a 

 different nature from the indefinite succession which takes place in 

 all the teeth continuously throughout the lifetime of the lower 

 vertebrates. 



When the milk-teeth are well developed, and continue in place 

 during the greater part of the animal's growth, as is especially the 

 case with the Ungulata, and, though to a less degree, with the 

 Primates and Carnivora, their use is obvious, since taken all together 

 they form structurally a complete epitome on a small scale of the 

 more numerous and larger permanent set (see Fig. 3), and, con- 

 sequently, are able to perform the same functions, while time is 

 allowed for the gradual maturation of the latter, and especially 

 while the jaws of the growing animal are acquiring the size and 

 strength sufficient to support the permanent teeth. Those animals, 

 therefore, that have a well-developed and tolerably persistent set of 

 milk-teeth may be considered to be in a higher state of development, 

 as regards their dentition, than those that have the milk-teeth 

 absent or rudimentary. 



It is a very general rule that individual teeth of the milk and 

 permanent set have a close relationship to one another, being 

 originally formed, as mentioned above, in exceedingly near proximity, 

 and with, at all events so far as the enamel-germ is concerned, a 

 direct connection. Moreover, since the latter ultimately come to 

 occupy the position in the alveolar border temporarily held by the 

 former, they are spoken of respectively as the predecessors or suc- 

 cessors of each other. But it must be understood that milk-teeth 

 may be present which have no successors in the permanent series, 



