DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 287 



As the first branchial arch develops, it grows ventrally until it meets and fuses 

 with its fellow of the opposite side in the midventral line, thus forming the 

 mandibular process. From the cephalic side of the first arch a secondary proc- 

 ess maxillary process develops and fills in the space between the arch itself 

 and the naso-frontal process. These various structures thus bound a distinct 

 depression on the ventral side of the head. This depression is the oral pit, the 

 forerunner of the oral and nasal cavities (Fig. 245; compare with Figs. 244 

 and 85). The groove in the midventral line between the mandibular pro- 

 cesses marks the symphysis of the lower jaws. The groove on each side 



Epiglottis 



Tongue 

 Hypophysis 



Larynx 



Lung 



... Stornaeh 



Pancreas 



/ ; - ~ -^ I-, | / l 



Caudal gut 



- Mesonephric duct 



Kidney bud 

 FIG. 246. Alimentary tube of a human embryo of 4.1 mm. His Kollmann. 



between the maxillary process and the mandibular process marks the angle of 

 the mouth. The groove between the maxillary process and the naso-frontal 

 process is the naso-optic furrow, at the dorsal end of which the eye develops. 

 The bottom of the oral pit is formed by a portion of the ventral body wall, 

 which separates the oral cavity from the cephalic end of the gut, and which is 

 composed of ectoderm and entoderm, with a small amount of mesoderm be- 

 tween. This closing plate, the pharyngeal membrane, which is still present in 

 I embryos of 2.15 mm., soon becomes thinner and finally breaks away, leaving 

 I the oral pit and the gut in direct communication (Fig. 247). Since the oral pit 

 , is lined with ectoderm, the epithelial lining of the mouth or oral cavity is largely of 



