ORGA 



DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 329 



lobes and lobules. The submaxillary grows the deeper of the two, and comes to 

 lie to the medial side of the ramus of the mandible. The sublingual comes to lie 

 beneath the mucosa at the side of the root of the tongue. The solid sprouts or 

 cords of cells acquire lumina which appear first at the point where the original 

 evagination joins the parent epithelium and gradually extend distally until the 

 terminal tubules -are formed. 



A little later than the first anlage of the sublingual gland, several other 

 evaginations develop from the epithelium of the floor of the mouth. These 

 develop in the same manner as the other glands and form a series of smaller 

 sublinguals. The latter and the primary sublingual are usually spoken of col- 

 lectively as the sublingual gland, although each has its own duct. Most of the 

 ducts open on the floor of the mouth near the submaxillary duct, a few open 

 into the latter. 



The ducts of the glands represent the proximal parts of the original evagina- 

 tions. The submaxillary (Wharton's) duct at first opens far back along the 

 root of the tongue, but during development it apparently migrates forward and 

 eventually opens at the side of the frenulum linguae. The sublingual (Bartho- 

 lin's) duct comes to open near the submaxillary. 



Some investigators hold that the parotid gland begins to develop during the 

 eighth week as a solid epithelial evagination from the corner where the floor and 

 roof of the mouth come together, that is, where the inferior and superior 

 alveolo-labial sulci join inside the cheek. Another view is that it appears 

 during the fourth week as the first of the salivary glands, the anlage being a 

 ridge-like evagination, and that the ridge breaks away from the parent epithe- 

 lium except at the anterior end which becomes the opening of the duct. In 

 either case the evagination grows through the connective (mesenchymal) tissue 

 of the cheek, and by- the tenth week lies across the masseter muscle. The distal 

 end branches freely to form the secreting portion of the gland, which comes to 

 lie in front of the ear and behind the ramus of the lower jaw. The proximal 

 portion of the evagination becomes hollow to form the parotid (Steno's) duct, the 

 intermediate portion to form the smaller ducts, and the distal branches become . 

 hollow (by the twelfth week) to form the terminal tubules. A small secondary 

 evagination from the main duct gives rise to the accessory parotid. 



The histogenetic changes in the salivary glands probably continue until the 

 child takes solid food, when the glands become of greater functional importance. 

 In the parotid gland, which is serous in man, the original, undifferentiated 

 epithelial cells undergo changes in form and arrangement so that by the 

 twenty-second week the larger ducts are lined with a two-layered epithelium, 

 the smaller ducts with a simple cuboidal epithelium, and the terminal tubules with 

 a single layer of high columnar cells. The two-layered epithelium in the larger 

 ducts persists. The ducts lined with the cuboidal epithelium become the 



