794 THE TEETH. 



period the papillce grow rapidly, they begin to show peculiarities of form, and 

 project from the mouths of the follicles. Soon, however, the follicles become 

 deeper, so as to hide the papillae, which now assume a shape corresponding 

 with that of the crowns of the future teeth. Small laminae or opercula of 

 membrane are then developed from the sides of each follicle, their number 

 and position being regulated, it is said, by the form of the cutting edges 

 and tubercles of the coming teeth : the incisor follicles having two laminae, 

 one external and one internal ; the canine, three, of which two are internal ; 

 and the molars, four or five each. The lips of the dental groove, as well as 

 the opercula, now begin to cohere over the follicles from behind forwards, 

 the posterior lip being very much thickened ; the groove itself ia thus 



Fig. 552. Fig. 552. ENLARGED VIEW OP THE UPPER AND 



LOWER DENT\L ARCHES OP A FCETUS OP ABOUT 

 FOURTEEN WEEKS. 



This specimen shows the follicular stage of develop- 

 ment of all the milk teeth as described by Goodsir ; 

 in each follicle the papilla is seen projecting ; but this 

 exposure of the papillae and the cavity of the follicles 

 probably arises from the accidental loss of the epithe- 

 lial covering. 



gradually obliterated, the follicles are con- 

 verted into close sacs, and the saccular stage 

 of the milk-teeth is thus completed about the 

 end of the fifteenth week. Certain lunated 

 depressions, which are formed one behind each 

 of the milk -follicles about the fourteenth week, 

 escape the general adhesion of the lips of the 



groove. From these depressions, as will b'e afterwards described, the sacs 

 of the ten anterior permanent teeth are subsequently developed. 



The first stages in the development of the teeth here described, the superficial origin 

 and open condition of the dental sacs, and the free papillary commencement of the 

 pulps, have been denied in recent years by Guillot, and by Robin and Magitot, who 

 assert that the sacs with their contents make their first appearance in the submucous 

 tissue, and are from the first closed sacs (Guillot in Annales des Sciences Naturelles, 

 vol. ix., 1859; Robin and Magitot in Journal de la Physiologic, 1860, vol. iii., pp. 

 130 and 663). The observations of Kolliker, however, seem to furnish a clue to the 

 explanation of what has been seen by these authors, at the same time that they con- 

 firm, in its most important features, Goodsir's mode of viewing the phenomena. In 

 the foetal lamb and calf, the first step in the formation of the tooth-germ, observed by 

 Kolliker, consists in a depression of a part of the deepest layer of the epithelium 

 into the subjacent mucous membrane. This depression, which, in common with 

 Huxley, he regards as the commencement of the foetal structure known as the enamel 

 organ, to be afterwards described, widens subsequently, so as to become flask-shaped, 

 remaining connected with the deep surface of the epithelium by a narrow neck. In 

 the next stage the dental papilla rising from the surface of the mucous membrane, 

 projects into, or indents the deepest side of the epithelial process or future enamel 

 organ, and the dental sac is formed at a somewhat later period in the surrounding 

 substance of the mucous membrane. In these animals, therefore, the epithelium of 

 the edge of the jaw covers in completely the enamel-germ or primary tooth-follicle. 



In man, Kolliker was unable to discover a similar arrangement, but found matters 

 very much, in the disposition described by Goodsir; that is, the follicles open, situated 

 in a dental groove of the jaw, and containing at their deepest part the dental papilke 

 developed from the mucous membrane. But he conceives it not improbable that in 

 Goodsir's specimens, as well aa in his own, the whole of the epithelium had been 

 abraded, and that the follicles and papillae were thus unnaturally opened to the surface. 



