492 



OF NUTRITION. 



Fig. 129. 





Upper Jaw of human embryo at sixth 

 week; showing 6, the primitive dental 

 groove, behind a, the lip. After Goodsir. 



rior Milk Molar tooth. About the eighth week, a 

 similar papilla, which is the germ of the Canine 

 tooth, arises in front of this ; and during the ninth 

 week the germs of the Incisors make their appear- 

 ance under the same form. Daring the tenth week, 

 processes from the sides of the dental groove, parti- 

 cularly the external one, approach each other, and 

 finally meet before and behind the papilla of the 

 anterior Molar; so as to enclose it in a follicle, 

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

 similar process, the other teeth are gradually en- 

 closed in corresponding follicles. The germ of the 

 Posterior milk Molar also appears during the tenth 

 week, as a small papilla. By the thirteenth week, 

 the follicle of the Posterior Molar is completed; and the several papillae undergo a gra- 

 dual change of form. Instead of remaining, as hitherto, simple, rounded, blunt masses 

 of granular matter, each of them assumes a particular shape; the Incisors acquire in 

 some degree the form of the future teeth; the Canines become simple cones; and the 

 Molars become cones flattened transversely, somewhat similar to carnivorous molars. 

 During this period, the papillae grow faster than the follicles ; 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 fourteenth 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 

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

 enclosed in as many follicles; and thus all necessary provision is made for the produc- 

 tion of the first set of teeth. (This series of changes is represented in Fig. 130, a g.) 

 The groove is now situated, however, on a higher level than at first; and it has under- 

 gone such a change by the closure of its edges, as to entitle it to the distinctive appella- 

 tion of secondary dental groove. It is in this secondary groove that those structures origi- 

 nate which are destined for the development of the Second or Permanent set of Teeth, 

 of those at least which replace the Milk Teeth. This is accomplished in the following 

 manner. 



Fig. 130. 



Diagrams illustrative of the formation of a temporary, and its corresponding permanent Tooth, from a 

 mucous membrane. After Goodsir. 



b. At about the fourteenth or fifteenth week, a little crescentic depression may be ob- 

 served immediately behind the inner Opercula of each of the Milk-tooth follicles. This 

 depression gradually becomes deeper, and constitutes what may be termed a cavity of 



