DEVELOPMENT OF THE TEETH. 793 



ing the borders of the maxilloe. About the sixth week of embryonic life, 

 a depression or groove, having the form of a horse- shoe, appears along 

 the edge of the jaw, in the mucous membrane of the gum ; this is the 

 primitive dental groove (Goodsir), From the floor of this groove (supposed 

 to be represented in a transverse section, in the diagrammatic figure 551,1) 



Fig. 551. 



/IS 1 



Fig. 551. DIAGRAMMATIC OUTLINES OP SECTIONS THROUGH THE DENTAL GERMS AND 

 SACS, AT DIFFERENT STAGES OF DEVELOPMENT (from Goodsir). 



1, the primitive dental groove of the gum cut across in a foetus of about six weeks; 2, 

 a papilla rising within the dental groove ; 3, 4, and 5, represent the follicular stage in 

 which the papilla (or future tooth-pulp) is seen sunk within the follicle, and the lips of 

 the follicle or opercula advancing towards each other gradually meet and close in the ful- 

 licle; 5, may be looked upon as representing the section indicated by the line a b, in fig. 

 559, through the sac of an incisor tooth, in which a lunated depression (c) is left behind; 

 in 6, the lips of the groove are seen to come together ; in 7, the union of the lips being 

 complete, the follicle becomes a closed sac s, containing the dental pulp p, and having 

 behind it the lunated depression c, now also enclosed, and forming the cavity of reserve 

 for the germ of the corresponding permanent tooth ; in the remaining outlines, 8 to 12, 

 are shown the commencement of the cap of dentine on the pulp, the subsequent steps in 

 the formation of the milk tooth, and its eruption through the gum (11); also the gradual 

 changes in the cavity of reserve, the appearance of its laminae and papilla, its closure to 

 form the sac of the permanent tooth, its descent into the jaw, behind and below the milk 

 tooth, and the long pedicle (12) formed by its upper obliterated portion. 



a series of ten papillae, as at 2, arise in succession in each jaw, and consti- 

 tute the germs or rudimentary pulps of the milk-teeth. These pulps or 

 papillae are processes of the mucous membrane itself, and not mere eleva- 

 tions of its epithelium. The order in which they appear is very regular. 

 The earliest is that for the first milk molar tooth : it is seen at the seventh 

 week, as soon as the dental groove is formed ; at the eighth week that for 

 the canine tooth appears ; the two incisor papillae follow next, at about the 

 ninth week, the central one before the lateral ; lastly, the second molar 

 papilla is visible at the tenth week, at which period this, the papillary stage 

 of the rudiments of the teeth is completed. The papillae in the upper jaw 

 appear a little earlier than those in the lower jaw. In the next place, the 

 margins of the dental groove become thickened and prominent, especially 

 the inner one ; and membranous septa or prolongations of the mucous mem- 

 brane pass across between the papillae from one margin to the other, so as 

 to convert the bottom of the groove into a series of follicles, each containing 

 one of the papillae. These changes constitute the follicular stage ; they take 

 place in the same order as that in which the papillae make their appearance, 

 and are completed about the fourteenth week. During the early part of this 



