DEVELOPMENT OF THE SALIVARY GLANDS 



161 



thelial ridge, the apex of the V corresponding to the site of the median thyreoid 

 evagination. At intervals along the epithelial ridges circular epithelial down- 

 growths develop which take the form of inverted and hollow truncated cones 

 (Fig. 153 B). During the fourth month circular clefts appear in the epithelial 

 downgrowths, 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 epi- 

 thelial 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. 



FIG. 153. Diagrams showing the development of the vallate papillae of the tongue (Graberg in 



McMurrich's "Human Body"). 



DEVELOPMENT OF THE SALIVARY GLANDS 



The glands of the mouth are all regarded as derivatives of the ectodermal 

 epithelium. Of the salivary glands, the parotid is the first to appear. Its anlage 

 has been observed in 8 mm. embryos as a furrow in the floor of the alveolo-buccal 

 groove. The furrow elongates and, in embryos of 17 mm., separates from the 

 epithelial layer, forming a tubular structure which opens into the mouth cavity 

 near the cephalic end of the original furrow. The tube grows back into the 

 region of the external ear, branches and forms the gland in this region, while the 

 unbranched portion of the tube becomes the parotid duct (Hammar, Anat. 

 Anzeiger, Bd. 19, 1901). 



The submaxillary gland arises as an epithelial ridge in the alveolo-lingual 

 groove, its cephalic end located caudal to the frenulum of the tongue. The 

 caudal end of the ridge soon begins to separate from the epithelium and extends 

 caudad and ventrad into the submaxillary region where it enlarges and branches 

 to form the gland proper, its cephalic unbranched portion persisting as the duct 

 which soon hollows out. 



The sub-lingual and alveolo-lingual glands develop as several solid evaginations of epi- 

 thelium from the alveolo-lingual groove, appearing from the eighth to the twelfth week (Fig. 



