DEVELOPMENT OP THE TEETH. 



291 



Fig. 83. 



the anterior superior "milk" Molar tooth. About the 8th week, a similar 

 papilla, which is the germ of the Canine tooth, arises in front of this ; and during 

 the 9th week, the germs of the Incisors make their appearance under the same 

 form. During the 10th week, processes from the sides of the dental groove, 

 particularly the external one, approach each other, 

 and finally meet before and behind the papilla of 

 the anterior Molar; so as to inclose it in a follicle, 

 through the mouth of which it may be seen. By a 

 similar process, the other teeth are gradually in- 

 closed in corresponding follicles. The germ of the 

 posterior " milk" Molar also appears during the 10th 

 week, as a small papilla. By the 13th week, the 

 follicle of the posterior Molar is completed ; and the 

 several papilla undergo a gradual change of form. 

 Instead of remaining, as hitherto, simple, rounded, 

 blunt masses of granular matter, each of them as- 

 sumes a particular shape ; the Incisors acquire in 

 some degree the flattened form of the future teeth ; the Canines become simple 

 cones j- and the Molars become cones flattened transversely, somewhat similar to 

 carnivorous molars. During this period, the papillae grow faster than the folli- 

 cles ; so that the former protrude from the mouth of the latter. At this time, 

 the mouths of the follicles undergo a change, consisting in the development of 

 their edges, so as to form opercula ; which correspond in some measure with the 

 shape of the crowns of the future teeth. There are two of these opercula in the 

 Incisive follicles, three for the Canines, and four or five for the Molars. At the 

 14th week, the inner lip of the dental groove has increased so much, as to meet 

 and apply itself in a valvular manner to the outer lip or ridge, which has been 

 also increasing. The follicles at this time grow faster than the papillae, so that 



Fig. 84. 



Upper jaw of Human Embryo at 

 sixth week ; showing b, the Primi- 

 tive Denial Groove, behind a, the Lip. 



Diagrams illustrative of the formation of a Temporary, and its corresponding Permanent Tooth 

 from a Mucous Membrane. 



the latter recede into the former. The primitive dental groove then contains 

 ten papillae, inclosed in as many follicles ; and thus all necessary provision is 

 made for the production of the first set of teeth. (This series of changes is 

 represented in Fig. 84, a //.) The groove is now situated, however, on a higher 



