328 TEXT-BOOK OF EMBRYOLOGY. 



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: 

 Upper Jaw Permanent, M. Pm. C. L. I. M. I. M. I. L. I. C. Pm. M. 



Upper Jaw Milk, M. C. L. I. M. I. 



211 



211 



Lower Jaw Milk, M. C.L.I. M.I. 



II II ll II 

 Lower Jaw Permanent, M. Pm. C. L. I. M. I. 



M.I. L.I. C. M. 



2 3 16 



2 3 16 



= _ = 32 



M.I. L.I. C. M. 



I! II II I! 1.1 



M.I. L.I. C. Pm. M. 



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). 



^_ Palatine process 



Subling. .,--. . 



Submax. gland 

 P::v-. ! rfc*.::..-.V.: 



Submax. glanj ' 



Lingual nerve / :.:' : 



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



The Salivary Glands. The anlage of the submaxillary gland on each side 

 appears in embryos of about 14 mm. as a solid evagination of the epithelium 

 at the side of the root of the tongue. A little later the first anlage of the sublin- 

 gual gland appears in the same region either as a solid outgrowth from the sub- 

 maxillary evagination or directly from the epithelium of the mouth. Each 

 anlage primarily grows into the floor of the mouth as solid sprouts of epithe- 

 lium (Fig. 293). The sprouts then branch repeatedly in the connective (mesen- 

 chymal) tissue, thus laying the foundation for the division of the glands into 



