DEVELOPMENT OF THE FACE. 



925 



At certain points these points corresponding to the situation of the true dental 

 bulbs there appear rounded enlargements at the free margin of the epithelial band- 

 just described. Each one of these is developed into one of the structures of the perfect 

 tooth. The mechanism of the formation of this, which is called the enamel-organ, and 

 of the dental bulb is as follows : 



Fro. 304. Mouth of a human embryon of thirty -five 

 days. (Coste.) 



1, frontal process widely sloped at its inferior portion ; 

 2, 2, incisor processes produced by this sloping; 3, 3, 

 nostrils; 4, lower lip and maxilla, formed by the 

 union of the inferior maxillary processes; 5, 5, supe- 

 rior maxillary processes contiguous to the incisor 

 process; 6, mouth, still confounded with the nasal 

 fossae; 7, first appearance of the closure of the nasal 

 fossae ; 8, 8, first appearance of the two halves of 

 the palatine arch; y, tongue ; 10, 10, eyes; 11,12, 

 13, visceral arches. 



FIG. 305. Mouth of an embryon of forty days. (Coste.) 

 1, first appearance of the nose; 2, 2, first appearance of 

 the alee of the nose ; 3, appearance of the closure be- 

 neath the nose ; 4, middle, or median portion of the up- 

 per lip, formed by the approach and union of the two 

 incisor processes, a little notch in the median line still 

 indicating the primitive separation of the two processes ; 

 5, 5, superior maxillary processes, forming the lateral 

 portions of the upper lip ; 6. 0. groove for the develop- 

 ment of the lachrymal sac and the nasal canal ; 7, lower 

 lip; 8, mouth; 9, 9, the two lateral halves of the pala- 

 tine arch, already nearly approximated to each other 

 in front, but still widely separated behind. 



A rounded enlargement appears at the margin of the epithelial band. This soon be- 

 comes directed downward (adapting our description to the lower jaw) and dips into the 

 mucous structure, being at first connected with the epithelial band by a narrow pedicle, 

 which soon disappears, leaving the enlargement enclosed completely in a follicle. This 

 is the dental follicle, and it has no connection with the wedge-shaped band which we de- 

 soribed first. While this process is going on, a conical bulb appears at the bottom of the 

 follicle. The enamel-organ, formed from the epithelial band, becomes excavated or cup- 

 shaped at its under surface and fits over the dental bulb, becoming united to it. 



The tooth, at this time, consists of the dental bulb, with the enamel-organ closely 

 fitted to its projecting surface. The enamel-organ is developed into the enamel ; the 

 dental bulb, which is provided with vessels and nerves, becomes the tooth-pulp ; and, 

 upon the surface of the dental bulb, the dentine, or ivory, is developed in successive 

 layers. The cement is developed by successive layers upon that portion of the dentine 

 which forms the root of the tooth. As these processes go on, the tooth projects more 

 and more, the upper part of the wall of the follicle gives way, and the tooth finally 

 appears at the surface. 



1 The periods of development indicated for these diagrams are somewhat earlier than those which we have 

 noted in the text ; but it Is impossible to fix these with absolute accuracy, and all the estimates given by authors are 

 understood to be merely approximative. 



