286 The Alligator and Its Allies 
entoderm around the mandibular folds (md); 
there it is somewhat thickened. The most strik- 
ing feature of the section is the presence of two 
large auditory vesicles (0). The section being 
not quite at right angles to this part of the embryo, 
the vesicles are not cut in exactly the same plane; 
the one on the left is cut through its opening to the 
exterior, while the one on the right appears as a 
completely enclosed cavity. In a section a short 
distance posterior to this one the appearance of 
the vesicles would be the reverse of what it is here. 
As may be seen in the figure, the vesicles are large, 
thick-walled cavities lying close to the lateral 
walls of the hindbrain. The hindbrain itself 
has the usual triangular cross-section, with thick 
lateral walls and a thin, wrinkled dorsal wall. 
Close to the ventral side of the hindbrain lies the 
notochord (nt), on each side of which, in the 
angle between the brain and the auditory vesicles, 
is a small blood-vessel (bv). Ventral to these 
structures and close to the dorsal wall of the 
pharynx (ph) are the two large dorsal aortz (ao). 
The ventral side of the section passes through the 
open anterior end of the pharynx (ph). On the 
left is seen the widely open hyomandibular cleft 
(g’), between the main body of the section and the 
mandibular arch (md). On the right side the 
plane of the section was such that the hyomandibu- 
lar cleft was not cut through its external opening. 
In each mandibular fold a large aortic arch (ar) is 
