DEVELOPMENT OF THE TEETH. 295 



Deciduous or milk teeth. 

 . 3-3 0-0 3-3_12_ 

 ^- 3^:3' ^' 0~0' '^' 3^~12"^*- 



Permanent teeth. 

 .3-3 1-1. 3-3 3-3 20 ,^ 



*• 3~3' '• rri' V-^""- 3-:r3' ^^- 3-3 = 20=*^- 



DEVELOPMENT OF TH3 TEETH. 



The development of the teeth, as demonstrated by the late 

 Professor Goodsir, consists of three distinct stages — papillary, 

 follicular, and saccular. About the sixth week of foetal life a depress- 

 sion is formed in the mucous membrane of the gum, which is the 

 primitive dental groove, from the floor of which arise papillae or 

 processes corresponding numerically to, and constituting the gerin^ 

 of, the milk teeth ; this is the papillary stage. About the tenth 

 week the groove deepens, the papillae enlarge, the margins of the 

 grooves thicken and become prominent ; prolongations or septa 

 pass from one side of the groove to the other, enclosing each 

 papilla in an open follicle or sac ; this constitutes the follicular 

 stage ; it terminates about the fourteenth week. Somewhat later, 

 the papillae begin to assume the shape of the crowns of the future 

 teeth. Small membranous processes or opercula are now developed 

 firom the sides of the follicle, which correspond in number and 

 shape with the table surfaces of the teeth ; thus, for the incisors, 

 there is one on either side, the tushes have three, and the molars 

 four or five, according to their size and situation. The lips of 

 the follicles, as well as the opercula, close and cohere, and the 

 grooves becoming obliterated, what were open follicles are con- 

 verted into closed sacs, thus completing the saccular stage. 

 Shortly before the closing of the follicles of the milk teeth, a 

 lunated depression is noticed behind, and to the inner aspect of 

 each follicle ; these become converted into cavities of reserve, 

 which ultimately form sacs for the development of the permanent 

 incisors, as well as the three anterior permanent molars. The 

 dental sac, as well as the enclosed papilla, continues to enlarge, 

 the papilla becoming converted into the pulp, and acquiring the 

 semblance of the dental crown. The contents of the sac now 

 consist of the vascular pulp, covered by its membrane — an internal 

 vascular layer, lining the sac, called the periodontal membrane ; 

 and a fluid lying between the two membranes, the outer or 



