1122 



SPLANCHNOLOGY 





ad. en. d. od. 



Development of the Deciduous Teeth. The development of the deciduous teeth begins 

 about the sixth week of fetal life as a thickening of the epithelium along the line of the future 

 jaw, the thickening being due to a rapid multiplication of the more deeply situated epithelial 

 cells. As the cells multiply they extend into the subjacent mesoderm, and thus form a ridge 

 or strand of cells imbedded in mesoderm. About the seventh week a longitudinal splitting or 

 cleavage of this strand of cells takes place, and it becomes divided into two strands; the separa- 

 tion begins in front and extends laterally, the process occupying four or five weeks. Of the two 

 strands thus formed, the labial forms the labiodental lamina; while the other, the lingual, 

 is the ridge of cells in connection with which the teeth, both deciduous and permanent, are 

 developed. Hence it is known as the dental lamina or common dental germ. It forms a flat 

 band of cells, which grows . into the substance of the embryonic j aw, at first horizontally 

 inward, and then, as the teeth develop, vertically, i. e., upward in the upper jaw, and 

 downward in the lower jaw. While still maintaining a horizontal direction it has two edges 

 an attached edge, continuous with the epithelium lining the mouth, and a free edge, projecting 

 inward, and imbedded in the mesodermal tissue of the embryonic jaw. Along its line of 

 attachment to the buccal epithelium is a shallow groove, the dental furrow. 



About the ninth week the dental lamina begins 

 to develop enlargements along its free border. 

 These are ten in number in each jaw, and each 

 corresponds to a future deciduous tooth. They 

 consist of masses of epithelial cells; and the cells 

 of the deeper part that is, the part farthest from 

 the margin of the jaw increase rapidly and spread 

 out in all directions. Each mass thus comes to 

 assume a club shape, connected with the general 

 epithelial lining of the mouth by a narrow neck, 

 embraced by mesoderm. They are now known as 

 special dental germs. After a time the lower ex- 

 panded portion inclines outward, so as to form an 

 angle with the superficial constricted portion, which 

 is sometimes known as the neck of the special 

 dental germ. About the tenth week the meso- 

 dermal tissue beneath these special dental germs 

 becomes differentiated into papillae; these grow 

 upward, and come in contact with the epithelial 

 cells of the special dental germs, which become 

 folded over them like a hood or cap. There is, 

 then, at this stage a papilla (or papillae) which 

 has already begun to assume somewhat the shape 

 of the crown of the future tooth, and from which 

 the dentin and pulp of the tooth are formed, sur- 

 mounted by a dome or cap of epithelial cells from 

 which the enamel is derived. 



In the meantime, while these changes have been 

 going on, the dental lamina has been extending 

 backward behind the special dental germ corre- 

 sponding to the second deciduous molar tooth, 

 and at about the seventeenth week it presents an 

 enlargement, the special dental germ, for the first 

 permanent molar, soon followed by the formation 



of a papilla in the mesodermal tissue for the same tooth. This is followed, about the sixth 

 month after birth, by a further extension backward of the dental lamina, with the formation 

 of another enlargement and its corresponding papilla for the second molar. And finally the pro- 

 cess is repeated for the third molar, its papilla appearing about the fifth year of life. 



After the formation of the special dental germs, the dental lamina undergoes atrophic changes 

 and becomes cribriform, except on the lingual and lateral aspects of each of the special germs 

 of the temporary teeth, where it undergoes a local thickening forming the special dental germ 

 of each of the successional permanent teeth i. e., the ten anterior ones in each jaw. Here the 

 same process goes on as has been described in connection with those of the deciduous teeth: 

 that is, they recede into the substance of the gum behind the germs of the deciduous teeth. As 

 they recede they become club-shaped, form expansions at their distal extremities, and finally 

 meet papillae, which have been formed in the mesoderm, just in the same manner as was the 

 case in the deciduous teeth. The apex of each papilla indents the dental germ, which encloses 

 it, and, forming a cap for it, becomes converted into the enamel, while the papilla forms the 

 dentin and pulp of the permanent tooth. 

 The special dental germs consist at first of rounded or polyhedral epithelial cells; after the 



ep.sch.-~~ :-v4" 



Flo. 1012. Longitudinal section of the lower part 

 of a growing tooth, showing the extension of the layer 

 of adamantoblasts beyond the crown to mark off 

 the limit of formation of the den tin .of the root. 

 (Rose.) ad. Adamantoblasts, continuous below with 

 ep.sch., the epithelial sheath of Hertwig. d. Dentin. 

 en. Enamel, od. Odontoblasts. p. Pulp. 



