650 



ORGANS OF DIGESTION. 



Development of Teeth. 

 Flo- 224. 



Frimiivvt JDtntal 



r\C. 325. 

 aJiitta,arG*rm. o 



. 327. 



FIG. 328. 



F I C . 329. 



The dental groove now becomes contracted, 

 its margins thickened and prominent, and the 

 groove is converted into follicles for the re- 

 ception of the papillae, by the growth of mem- 

 branous septa, which pass across the groove 

 between its borders (fig. 326). The follicles 

 by this means become the alveoli, lined by 

 periosteum, from the bottom of which a process 

 of the mucous membrane of the gum rises, 

 which is the germ of the future tooth. The 

 follicle for the first molar is complete about the 

 tenth week ; the canine follows next, succeeded 

 by the follicles for the incisors, which are com- 

 pleted about the eleventh or twelfth week ; and, 

 lastly, the follicle of the posterior deciduous 

 molar is completed about the fourteenth week. 

 These changes constitute the second or follicu- 

 lar stage. 



About the thirteenth week, the papilla? begin 

 to grow rapidly, project from the follicles, and 

 assume a form corresponding with that of the 

 future teeth ; the follicles soon become deeper, 

 and from their margins small membranous 

 processes or opercula are developed, which, 

 meeting, unite and form a lid to the now closed 

 cavity (fig. 327). These processes correspond 

 in shape to the form of the crown of the 

 tooth, and in number to the tubercles on its 

 surface. The follicles of the incisor teeth have 

 two opercula, the canine three, and the molars 

 four or five each. The follicles are thus con- 

 verted into dental sacs, and the contained 

 papilla become pulps. The lips of the dental 

 groove gradually advance over the follicles from 

 behind forwards, and, uniting, gradually ob- 

 literate it. This completes the third or saccular 

 stage, which takes place about the end of the 

 fifteenth week. 



The deep portion of the primitive dental 

 groove is now closed-in; but the more super- 

 ficial portion, near the surface of the gum, 

 still remains open ; it is called, by Mr. Goodsir, 

 the secondary dental groove; from it are 

 developed the ten anterior permanent teeth. 

 About the fourteenth week, certain lunated 

 depressions are formed, one behind each of 

 the sacs of the rudimentary milk-teeth. They 

 are ten in number in each jaw, and are 

 formed successively from before backwards; 

 they are the rudimentary follicles of the four 

 permanent incisors, the two canine, and the 

 four bicuspids. As the secondary dental groove 

 closes in, these follicles become closed cavities of 

 reserve (fig. 327). The cavities soon elongate, 

 and recede from the surface into the substance 

 of the gum, behind the sacs of the deciduous 

 teeth, and a papilla projects from the bottom of 



