rULPS AND SACS OF THE HUMAN TEETH. 29 



During the eleventh and twelfth week the incisives ad- 

 vance steadily. Septa pass between them from the outer to 

 the inner side of the groove, so that their papilla; become 

 completely sunk in well-developed follicles. Xo material 

 change takes place in the anterior molar or canine ; but the 

 posterior molar papilla enlarges, and the terminal lobule of 

 the outer ridge folds gradually round it, so as to constitute its 

 follicle, behind which there still remains a portion of the 

 primitive groove. 



The changes which ensue during the thirteenth week con- 

 sist in the completion of the follicle of the posterior molar, 

 and in the gi'adual change which takes place in the shape of 

 the different papilla?. Instead of remaining, as hitherto, simple, 

 rounded, blunt masses of granular matter, each of them assumes 

 a particular shape. The incisives acquire in some degree the 

 appearance of the future teeth ; the canines become simple 

 cones ; and the molars become cones flattened transversely, 

 somewhat similar to carnivorous molars. During this period, 

 too, the papillae grow faster than the follicles, so that the for- 

 mer protrude from the mouth of the latter, while the depth of 

 the latter varies directly as the length of the fangs of their 

 future corresponding teeth, the canine follicle being deepest, 

 etc. etc. AMiile the papillne are changing their shape, the 

 mouths of the follicles are undergoing a change which con- 

 sists 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, one larger, anterior, and rather ex- 

 ternal, the second smaller, posterior, and internal. There are 

 three for the canines, an external and two internal, and four 

 or five for the molars, each corresponding with a tubercle ; 

 while their edges correspond with the grooves on the grinding 

 surfaces of these teeth. 



• It would be interesting to ascertain whether the opercula of the human 



