66 DEVELOPEMENT OF TEETH. 



a slight bulging at the seventh week, and the tenth papilla is not 

 apparent until the eleventh week. 



From about the eighth week the primitive dental groove becomes 

 contracted before and behind the first deciduous molar, and laminae 

 of the mucous membrane are developed around the other papillae, 

 which increase in growth and enclose the papillse in follicles with 

 open mouths. At the tenth week the follicle of the first molar is 

 completed, then that of the canine ; during the eleventh and twelfth 

 weeks the follicles of the incisors succeed, and at the thirteenth 

 week the follicle of the posterior deciduous molar. 



During the thirteenth week the papillae undergo an alteration of 

 form, and assume the shape of the teeth they are intended to repre- 

 sent. And at the same time small membranous processes are de- 

 veloped from the mouths of the follicles ; these processes are intended 

 to serve the purpose of opercula to the follicles, and they correspond 

 in shape with the form of the crowns of their appertaining teeth. 

 To the follicles of the incisor teeth there are two opercula ; to the 

 canine, three ; and to the molars a number relative to the number 

 of their tubercles, either four or five. During the fourteenth and 

 fifteenth weeks the opercula have completely closed the follicles, so 

 as to convert them into dental sacs, and at the same time the papillae 

 have become pulps. 



The deep portion of the primitive dental groove, viz. that which 

 contains the dental sacs of the deciduous teeth, being thus closed in, 

 the remaining portion, that which is nearer the surface of the gum, 

 is still left open, and to this Mr. Goodsir has given the title of 

 secondary dental groove ; as it serves for the developement of all the 

 permanent teeth, with the exception of the anterior molars. During 

 the fourteenth and fifteenth weeks small lunated inflections of the 

 mucous membrane are formed, immediately to the inner side of the 

 closing opercula of the deciduous dental follicles, commencing 

 behind the incisors and proceeding onwards through the rest; these 

 are the rudiments of the follicles or cavities of reserve of the four 

 permanent incisors, two permanent canines, and the four bicuspides. 

 As the secondary dental groove gradually closes, these follicular 

 inflections of the mucous membrane are converted into closed 

 cavities of reserve, which recede from the surface of the gum and lie 

 immediately to the inner side and in close contact with the dental 

 sacs of the deciduous teeth, being enclosed in their submucous cel- 

 lular tissue. At about the fifth month the anterior of these cavities 

 of reserve dilate at their distal extremities, and a fold or papilla 

 projects into their fundus, constituting the rudiment of the germ of 

 the permanent tooth ; at the same time two small opercular folds 

 are produced at their proximal or small extremities, and convert 

 them into true dental sacs. 



During the fifth month the posterior part of the primitive dental 

 groove behind the sac of the last deciduous tooth has remained 

 open, and in it has developed the papilla and follicle of the first per- 

 manent molar. Upon the closure of this follicle by its opercula, 



