CII. V.] 



DEVELOPMENT OF THE TEETH 



75 



the tooth (as is especially well seen in the abnormal condition called 

 an exostosis), by the process of ossification taking place in the perios- 

 teum. On the other hand, absorption of the root (such as occurs 

 when the milk-teeth are shed) is due to the action of the osteoclasts 

 of the same membrane. 



In this manner the first set of teeth, or the milk-teeth, are formed ; 

 and each tooth, as it grows, presses at length on the wall of the 

 sac enclosing it, and, causing its absorption, is cut, to use a familiar 

 phrase. 



The temporary or milk-teeth 'are later replaced by the growth of 

 the permanent teeth, which push their way up from beneath them. 







FIG. 101. Highly magnified view of a piece of the enamel organ in a kitten's canine, d, Superficial 

 layer of dentine, e, Newly formed enamel stained black by osmic acid. T, Tomes' processes from 

 the adamantoblasts, ad. ; str. int., stratum intermedium of the enamel organ, p, Branched cells of 

 the enamel pulp. (After Rose.) 



Each temporary tooth is replaced by a tooth of the permanent set 

 which is developed from a small sac which was originally an offshoot 

 from the sac of the temporary tooth which precedes it, and called the 

 cavity of reserve (fig. 98, c, fp). Thus the temporary incisors and 

 canines are succeeded by the corresponding permanent ones, the 

 temporary first molar by the first bicuspid ; the temporary second 

 molar develops two offshoots, one for the second bicuspid, the other 

 for the permanent first molar. The permanent second molar is budded 

 off from the first permanent molar, and the wisdom from the perma- 

 nent second molar. 



The development of the temporary teeth commences about the 

 sixth week of intra-uterine life, after the laying down of the bony 

 structure of the jaws. Their permanent successors begin to form 



