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 aortas (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 f ) , 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 



