DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 319 



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 

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

 forerunner of the oral and nasal cavities (Fig. 283; compare with Figs. 282 

 and 122). The groove in the midventral line between the mandibular processes 

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



Epiglottis 



Tongue 

 Hypophysis ./. 



Laryi 



M Lung 



EM- \ Stomach 



Pancreas 



Caudal gut ...--i- 



Urachus 



Mesonephric duct 



Kidney bud 



FIG, 284. Alimentary tube of a human embryo of 4.1 mm. His Kollmann, 



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 

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

 the oral pit and the gut in direct communication (Fig. 285). Since the oral pit 

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



