DEVELOPMENT OF THE TEETH. 295 



Deciduous or milk teeth. 



3Q An o q i o 



o u u o o j..i . 



Permanent teeth. 



3 



-3 



3-3 3-3 20.. 

 p.m. , m. = - = 40. 



DEVELOPMENT OF THE 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 

 depression 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 germs of, the milk teeth ; this is the papillai*y stage. About 

 the tenth week the groove deepens, the papillae enlarge, the 

 margins of the grooves thicken and become prominent ; prolonga- 

 tions 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 from 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 converted 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 7 

 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 



