S36 MAMMALS. 



the vascular mesodermic dermis is defined off as the " den- 

 tine germ." The crown of this papilla becomes hard, and 

 the ossification proceeds downwards and inwards, while 

 above the dentine crown the enamel begins to form a hard 

 cap. Meantime the tissue around the base of the tooth- 

 papilla becomes differentiated into an enclosing foUicle or 

 sac, from the inner layer of which the cement is developed. 

 The pulp is but the uncalcified core of the papilla. When 

 there are two sets of teeth, the enamel-germ which begins 

 a tooth of the second set is a bud from the enamel-germ of 

 its predecessor of the first set. Similarly in regard to the 

 molars, of which there is never more than one set, the first 

 molar has an independent origin, the enamel-germ of each 

 of the others is budded from the molar immediately in front 

 of it. 



The base of a tooth may remain unconstricted, and the 

 core of pulp may persist. Such a tooth goes on growing, 

 its growth usually keeping pace with the rate at which the 

 apex is worn away with use, and it is described as " root- 

 less " and " with persistent pulp." The incisors of Rodents 

 and of Elephants illustrate this condition. 



In the development of most teeth, however, the base is 

 narrowed and prolonged into a root or several roots which 

 become firmly fixed in the socket. Through a minute 

 aperture at the end of the root, blood-vessels and nerves 

 still enter the pulp-cavity and keep the tooth alive, but as 

 the limit of growth is reached the residue of soft pulp tends 

 to disappear. Of these "rooted" teeth there are many 

 kinds, differing in size and shape, in the number of 

 roots, and in the period at which these are definitely estab- 

 lished. Mammals also differ not a little in regard to the 

 period at which the teeth — ^usually concealed at the time of 

 birth — appear on the surface or cut the gum. 



Some mammals have but one set of teeth. If one of 

 them be lost or worn away, it cannot be replaced. These 

 mammals, such as the sloths and the toothed whales, are 

 called monophyodont. But most mammals have two sets 

 of teeth, — a more important permanent set which is func- 

 tional through the greater part of life, and a less important 

 transient first set the members of which, often being de- 

 veloped during the period of sucking, are called milk- 



