PLATE Vn {ronlluml). 



fangs are compressed from side to side, the central stem of the dentinal 

 pulp is not seen in this seetion, and the enamel organ passes entirely round 

 its lateral expansion. The dentinal pulp itsrlf is not re|U-esent.ed in cither 

 of the two teetli ; two contour-lines, bounding the apical h;df of the space 

 which it occupied in the larger of the two teelli, show the extent to which 

 the deposition of enamel and dentine severally had proceeded upon it. In 

 the smaller of the two teeth flie deposition of enamel has not commenced, 

 and the enamel organ has as yet suffered no diminution of its " spongy," or 

 "gelatinous," or "stellate" tissue. This drawing being semi-diagram- 

 malic, segments only of the histological elements making up the epithelium 

 of the gum, the epidermis of the lip, the tooth-sac, and the enamel organs, 

 have been given; the contour-lines prolonged in each case from the external 

 boundaries of these segments, appearing to indicate sufficiently the relations 

 held in nature by the several structures. 



a. Anterior surface of lip. 



b. Epidermis of lip. 



c. Epithelium of gum. 



d. Tooth-sac, which at this stage in the development of the tooth, and 



before it receives any suppert from the bony structures in the jaw, is 

 clearly marked oif by layers of condensed cellular tissue from the 

 strata of cutis vera, which arc interposed between it and the external 

 epidermis. The loose spongy central portions of the tooth-sac bear 

 some resemblance, when viewed with the unassisted eye, to the 

 similarly placed stellate element of the enamel organ ; they differ 

 from it, however, by being vascular, and even highly vascular ; 

 whilst they differ from the cutis vera, not merely by their greater 

 looseness of texture and their greater vascularity, but also, as seen 

 in the figure, by the absence of glands, of hair- bulbs, and of mus- 

 cular tissue. 



e. Enamel organ. From the point to which the line e is drawn, down- 



wards, the enamel organ of the larger tooth is seen to possess all the 

 three structures; viz., the inner epithelium, the stellate or spongy 

 tissue, and the outer epithelium, which the enamel organ of the 

 smaller tooth (/<') still possesses. Above the point to which the 

 line e is drawn, the stellate tissue has disappeared, and the two 

 layers of the enamel organ's epithelium have come into apposition. 

 Thus the epithelial cells of the inner layer, which produce the enamel 

 prisms, or " fibres," come into closer relation with the blood-vessels 

 of the tooth capsule, whence alone, in the absence of vessels in the 

 enamel organ, they can provide themselves with the requisite mineral 

 matter. 



/. Space in the larger tooth occupied by the laterally projecting portion 

 of the spoon-shaped dentinal pulp. 



p. Corresponding space in the smaller tooth : in neither tooth did the 

 central stem of dentine come into view in this section. 



g^. Contour-line indicating the extent to which the deposition of enamel 

 has proceeded in the larger tooth. This line corresponds to the 

 similarly lettered granular deposit in fig. 1, Internally to this line 

 a second line is seen describing a similar contour, but reaching 

 considerably further down. It indicates the extent to which the 

 cap of dentine reaches downwards upon the exterior of the pulp; 

 this extent being considerably greater (as is seen also in fig. 1) 

 than that attained to by the deposit of enamel at this period of 

 development. 



h and //'. Line of junction, in the larger and smaller tooth respectively, 

 of the stellate tissue of the enamel organ to its inner layer of 

 epithelium. In both enamel organs the outer layer of epithelium is 

 drawn as more nearly columnar than it is in nature. 



