322 The Alligator and Its Allies 
Figure 20d 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 (né), 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 (pz) 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 otherend. 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 20e is only a short distance posterior to 
the preceding. On the left side the pharynx (ph) 
is connected with the exterior through the stomo- 
dzeum, and on the right the hyomandibular cleft 
(g’) iscut almost through its opening to the exterior. 
