322 TEXT-BOOK OF EMBRYOLOGY. 



in the region of the second and third branchial arches, gradually enlarge and 

 unite with each other and with the tuberculum impar, leaving between the 

 latter and themselves, however, a V-shaped groove (Fig. 289). At the apex of 

 the groove there is a depression — the foramen ccecum lingua — which is the ex- 

 ternal opening of the thyreoglossal duct (see p. 333). The groove later disap- 

 pears, but its position is indicated in the adult by the vallate papillae. 



According to Hammar, the tuberculum impar is a transitory structure and does not 

 give rise to the tip and body of the tongue. The tip and body are derived from a much 

 more extensive elevation in the floor of the pharynx. 



The tongue as a whole enlarges and grows from its place of origin toward 

 the entrance to the primary oral cavity. For a time it practically fills the cavity. 

 When the palate develops it recedes and finally comes to lie on the floor of the 

 oral cavity proper, as in the adult. The growth of the tongue involves the 



- — Tuberculum impar 



Fig. 288. — Floor of pharyngeal region of a human embryo of 12.5 mm. His, 



epithelial lining of the pharynx and oral cavity and also the underlying mesen- 

 chymal tissue. The latter produces the connective tissue and at least a part of 

 the intrinsic muscle fibers of the tongue. The papillae involve the epithelium 

 and connective tissue, while the glands and taste buds are derived from the 

 epithelium alone. 



The portion of the lingualis muscle innervated by the facial (VII) nerve is probably 

 derived from the mesenchymal tissue in the tongue anlage. The rest of the muscle is 

 innervated by fibers from the hypoglossal (XII) nerve, indicating a possible derivation from 

 certain rudimentary segments in the occipital region which correspond to the three roots of 

 the nerve. This would make it appear that during phylogenesis a part of the lingualis 

 muscle has grown into the tongue from a region caudal to the last branchial arch 



The lingual papilla begin to develop during the third month. Their 

 development is limited to the dorsum of the tongue and to the portion derived 

 from the tuberculum impar. In other regions slight elevations may appear, but 

 not in the form of distinct papillae. The fungiform and filiform papillae appear 

 as pointed elevations in the connective tissue, which push their way into the 

 epithelium, the latter at the same time being raised above the surface over these 



