322 The Alligator and Its Allies 



Figure 2od represents a section through the 

 hindbrain (hb), pharynx (ph), and tip of the snout. 

 On either side of the hindbrain are a convoluted au- 

 ditory vesicle (0), and several blood-vessels and 

 nerves, while ventral to it is seen the anterior end 

 of the notochord (nt) , around which the mesoblast 

 is somewhat more dense than elsewhere. The 

 pharynx (ph) sends out toward the surface a 

 narrow gill cleft (g') in the neighborhood of each 

 auditory vesicle. These clefts connect with the 

 exterior by very narrow slits, not seen in the plane 

 of this section. The opposite end of the pharynx, as 

 seen in this figure, opens on the left (pn) into the 

 nasal chamber. The nasal cavity on the right is 

 cut in such a plane that it shows neither its external 

 nor its pharyngeal opening. The nasal passages 

 are here fairly long and nearly straight chambers; 

 their lining epithelium is quite thick in the middle 

 region, but becomes thinner where it merges into 

 the epithelium of the pharynx at one end, and into 

 the superficial epithelium at the other end. The un- 

 usual appearance of the eye (e) , on the right side of 

 the figure, is due to the fact that the plane of the 

 section cut tangentially through the extreme edge 

 of the eye in the region of the choroid fissure. 



Figure 2Oe is only a short distance posterior to 

 the preceding. On the left side the pharynx (ph) 

 is connected with the exterior through the stomo- 

 dasum, and on the right the hyomandibular cleft 

 (g') is cut almost through its opening to the exterior. 



