138 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



The mucous membrane is continuous with the skin 

 at the outer border of the lips. At this border the 

 horny layer of skin begins, otherwise the skin and 

 this mucous membrane are similar structures. 



Morphologically, the mouth cavity is to be re- 

 garded as a part of the outside surface of the body, 

 which, embryologically, has been included by the 

 development of neighboring parts. At the time the 

 neural folds are closing dorsally to form the brain and 

 cord there develops a series of paired, ventral, 

 facial pits. These, enumerated from before back- 

 wards, are: the lens of the eye, the nasal pit, the 

 mouth, and gill clefts. The tissue between the latter 

 are called visceral arches, while that one between the 

 anterior gill cleft and the mouth cavity is the man- 

 dibular arch. The latter is morphologically analogous 

 to the visceral arches. In man the gill clefts all 

 finally close permanently, but the ectodermal em- 

 bryonic mouth cavity ultimately unites with the 

 embryonic foregut, thus forming the fauces which 

 lead to the pharynx. This final perforation be- 

 tween the mouth cavity and the foregut is paired, in 

 lower forms, which with other embryonic relations 

 confirms the view that the mouth cavity morpho- 

 logically represents a median fusion of two gill clefts. 



During this period of development the forebrain 

 grows ventrally and the mandibular arch grows in the 

 same direction. The space between these structures 

 is the beginning of the mouth and the nose, and is 

 called the stomodeum. At this time a rounded eleva- 

 tion, from the base of the mandibular arch, grows for- 

 ward along the base of the forebrain. This growth 



