DEVELOPMENT OF THE TEETH. 969 







oral nor an anal aperture. The buccal orifice is originally formed by 

 a depression above the first branchial arch, and then opens into the 

 pharynx, the tongue being already developed in its interior. At the 

 lower end of the rectum, the anal orifice appears as a depression, 

 which ultimately opens into the bowel. The stomach proper is, at 

 first, a longitudinal dilatation of the alimentary tube, which gradually 

 assumes an oblique, and then a transverse position. The primitive 

 alimentary tube is closely attached to the vertebral column, and is 

 covered by the peritoneum formed upon it and upon the walls of the 

 cavity of the body, as it separates from the latter. But after the 

 stomach has changed its position, the convolutions of the small intes- 

 tine and the remarkable bend of the large intestine around them 

 occur. These changes are owing to a greater development of the in- 

 testine than of the mesentery. This latter structure and the omenta 

 are now fully formed. The small intestine is, for a time, wider than 

 the large intestine. The vermiform appendix of the caecum is, as it 

 were, an incompletely developed, yet growing part of the large intes- 

 tine. The valvulae conniventes of the' small intestine and the sacculi 

 of the colon, appear afterwards. Fringed villi, at first, exist through- 

 out the embryonic alimentary canal, but they are permanent only in 

 the small intestine. 



The Teeth. 



In the cavity of the mouth, the middle and internal germinal layers 

 give origin to the buccal papillae and also to the teeth, which are them- 

 selves formed, partly by the corium, and partly by the epithelium of 

 the buccal mucous membrane. At first, the rudimentary upper and 

 lower jawbones of Man have no alveoli, and the membrane which 

 covers their horseshoe shaped borders is quite smooth. After a time, 

 however, a groove appears on the margin of each maxillary bone, 

 which gradually deepens and widens, and becomes separated by thin 

 osseous septa into rudimentary alveoli. 



In the meantime, according to one authority (Goodsir), the mucous 

 membrane over the margin of the jaws also presents a groove, called 

 the primitive dental groove, from the bottom of which minute papillae 

 arise, in the human jaw ten in number, above and below. These are 

 the rudimentary pulps of the future milk teeth. Those of the upper 

 jaw appear first. In each, the order of their appearance is as follow: 

 the first molar, the canine, the central incisor, the lateral incisor, and 

 the second molar. This is the papillary stage, which is soon converted 

 into thefotticular stage, by the rising up of membranous folds between 

 and around the papillae. By this time each papilla has enlarged, and 

 assumed the form of the crown of the future tooth ; whilst small mem- 

 branous lids, or opercula, corresponding, in number and shape, to the 

 surfaces of each' tooth, overlap the papilla. Subsequently these folli- 

 cles become deeper, and are closed by the adhesion of their opercula 

 and by the union of the borders of the dental groove, and, at their 

 upper part, a thicker portion is seen, which constitutes the enamel 

 organ. The so-called dental sacs are thus formed, and the saccular 

 stage is completed. 



