DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 291 



points. Gradually the little masses of connective tissue assume the shapes 

 characteristic of fungiform or filiform papillae. During the fifth month 

 the epithelium between the papillae apparently degenerates to some extent, 

 thus leaving them projecting still farther above the surface. The forma- 

 tion of papillae probably goes on for some time after birth, since at birth their 

 form, size, number and arrangement are not the same as at later periods. It is 

 an interesting fact that the filiform papillae lose many of their taste buds after 

 the child is weaned. 



The anlage of the vallate papillae appears as a ridge along the V-shaped line 

 of fusion between the paired and unpaired portions of the tongue. The ridge is 

 apparently formed by the ingrowth of a solid mass of epithelium along each 

 side, although the connective tissue between the masses may grow toward the 

 surface to some extent. Later the ridge is broken up into the individual papillae 



Tuberculum impar 



Root of tongue 



FIG. 251. Dorsal view of the tongue of a human embryo of 20 mm. His, Bonnet. 



by the ingrowth of the epithelium at certain points. The more superficial cells 

 of the masses then degenerate, thus leaving each papilla surrounded by a trench 

 and wall. 



The development of the lingual glands is confined for the most part to the 

 root and inferior surface and to the region of the vallate papillae. The glands 

 begin to develop during the fourth month as solid ingrowths of epithelium, the 

 mucous glands appearing first, the serous somewhat later. The epithelial 

 masses acquire lumina and grow deeper into the tongue, where they usually 

 branch and coil to form the secreting portions. The latter open to the surface 

 through the original ingrowths which become the ducts. Ebner's glands 

 develop from the bottoms of the trenches around the vallate papillae. 



The Teeth. The development of the teeth involves the ectoderm and 

 mesoderm, the former giving rise to the enamel, the latter to the dentine and 

 pulp. In human embryos of 12-15 mm. (thirty-four to forty days), before 

 the lip groove is formed, a thickening of the epithelium (ectoderm) takes place 



