DEVELOPMENT OF THE SALIVARY GLANDS 153 



The foregoing account applies to the early origin of the mucous membrane alone. The 

 musculature of the tongue is supplied chiefly by the hypoglossal nerve, and both nerve and 

 muscles belong historically to the postbranchial region. If not in the development of each 

 present-day embryo, at leasb in the past the musculature has migrated cephalad and invad- 

 ed the branchial region beneath the mucous membrane (cf. p. 320). At the same time, the 

 tongue may be said to extend caudad until its root is covered by the epithelium of the 

 third and fourth branchial arches. This is shown by the fact that the sensory portions 

 of the nn. trigeminus andfacialis, the nerves of the first and second arches, supply the body 

 of the tongue, while the nn. glossopharyngeus and vagus, the nerves of the third and fourth 

 arches, supply the root and the caudal portion of the body of the tongue. 



Anomalies. Faulty development or incomplete fusion of the several anlages causes 

 variable degrees of absence or bifurcation of the tongue. 



In fetuses of 50 to 60 mm. (CR) the fungiform and filiform papilla; 

 may be distinguished as elevations of the epithelium. Taste buds appear 

 in the fungiform papillae of 100 mm. (CR) fetuses and are much more 

 numerous in the fetus than in the adult. The vallate papilla (Fig. 158 A) 

 develop on a V-shaped epithelial ridge, the apex of the V corresponding to 



A B C 



FIG. 158. Diagrams showing the development of the vallate papillae of the tongue (Graberg 

 in McMurrich). a, Valley; b, von Ebner's gland. 



the site of the thyreoid evagination (foramen caecum). At intervals along 

 the epithelial ridges, circular epithelial downgrowths occur (85 mm. CR) 

 which take the form of inverted and hollow truncated cones (Fig. 158). 

 During the fourth month circular clefts appear in the epithelial down- 

 growths, thus separating the walls of the vallate papillae from the surround- 

 ing epithelium and forming the trench from which this type -of papilla 

 derives its name. At the same time, lateral outgrowths arise from the 

 bases of the epithelial cones, hollow out and form the ducts and glands of 

 Ebner. The taste buds of the vallate papillae are also formed early, 

 appearing in embryos of three months. Foliate papilla probably develop 

 at about six months. 



DEVELOPMENT OF THE SALIVARY GLANDS 



The glands of the mouth are all regarded as derivatives of the ecto- 

 dermal epithelium. They complete their differentiation only after birth. 



The parotid is the first to appear. Its anlage has been observed in 

 8 mm. embryos, near the angle of the mouth, as a keel-like flange in the 

 floor of the alveolo-buccal (i.e., jaw-cheek) groove (Hammar). The 



