324 The Alligator and Its Allies 



bulbus is nearly circular in outline, though its cav- 

 ity is very irregular. A few sections anterior to this, 

 the opening of the bulbus into the ventricle is seen. 



In Figure 20g the section represented is only a 

 short distance posterior to the one represented by 

 Figure 20/. The mesoblastic structures in the 

 neighborhood of the spinal cord (sc) and notochord 

 (;z/) will be described in connection with the next 

 figure, where they are more clearly defined. The 

 oesophagus (oe) — or posterior end of the pharynx, 

 whichever it may be called — is here a crescentic 

 slit, with its convex side upward ; ventrally it opens 

 by a narrow glottis into the trachea (to). The 

 trachea is surrounded by the same condensed area 

 of mesoblast (la) that was mentioned in connection 

 with the preceding figure, but the condensation is 

 here more marked. From the bulbus (b) an aortic 

 arch (ar) extends upward for a short distance on the 

 right side, while to the left of the oesophagus an aor- 

 tic arch(ar) is cut through the upper part of its course. 

 Ventral to the bulbus the ventricle (vn) and two auri- 

 cles (au) are seen surrounded by the pericardial wall. 



Figure 2oh is in the region of the liver (li), which 

 has about the same position in relation to the auri- 

 cles (au) that was occupied by the ventricle in the 

 last figure. The auricles are connected with 

 each other by a wide passage. The trachea (ta) 

 and the oesophagus (oe) are entirely distinct from 

 each other; the former is a small, nearly circular 

 hole, while the lumen of the latter is obliterated and 



