DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 327 



advanced (embryos of 180 mm., about seventeen weeks). The permanent 

 incisors and canines appear about the twenty-fourth week; the premolars, which 

 correspond to the milk molars, about the twenty-ninth week. The second 

 molar does not appear till after birth (six months), and the third molar, or 

 wisdom tooth, begins to develop about the fifth year. 



The formation of the anlagen of the permanent teeth and the development of 

 the enamel, dentine and pulp take place in precisely the same manner as in the 

 milk teeth. The true molars grow out through the gums in the same way as 

 the milk teeth. Those permanent teeth which correspond in position to milk 

 teeth grow under the latter, exert pressure on their roots and thus loosen and 

 finally replace them. The two sets of teeth may be graphically represented 

 thus: 



16 



Normally all the epithelium of the dental shelf, except the parts directly con- 

 cerned in the development of the teeth, disappears at times which vary in differ- 

 ent individuals. Occasionally, however, remnants of this epithelium give rise 

 to cystic structures (developmental tooth tumors). 



:^' : -_- Tongue 

 P. 



Subling. gland 



Submax. gland 



Palatine process 



Submax. gland 



Lingual nerve 

 FIG. 293. From a transverse section through the tongue and oral cavity of a mouse embryo. Goppert, 



The Salivary Glands. The anlage of the submaxillary gland appears, in 

 embryos of 10 to 12 mm., as a flange of epithelium directed ventrally from 

 the portion of the lingual sulcus just caudal to the crossing of the lingual 

 nerve. The flange grows into the mesenchyme of the lower jaw, and at an 

 early period becomes triangular with its longest side free and a free vertical 

 caudal border. Cell proliferation begins at the angle of union of the two 

 borders and gradually progresses cephalad along the longest border, thus 

 producing a solid ridge-like thickening of the latter. 



