324 The Alligator and Its Allies 
bulbus is nearly circular in outline, though its cav- 
ity is very irregular. A fewsectionsanterior to this, 
the opening of the bulbusintothe ventricleis seen. 
In Figure 20g the section represented is only a 
short distance posterior to the one represented by 
Figure 20f. The mesoblastic structures in the 
neighborhood of the spinal cord (sc) and notochord 
(nt) will be described in connection with the next 
figure, where they are more clearly defined. The 
cesophagus (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 (ta). The 
trachea is surrounded by the same condensed area 
of mesoblast (Ja) that was mentioned in connection 
with the preceding figure, but the condensation is 
here more marked. From the bulbus (0) anaortic 
arch (ar) extends upward for a short distance on the 
right side, while to the left of the oesophagus an aor- 
ticarch(ar)is cut through the upper part ofits course. 
Ventral to the bulbus the ventricle (vv) and two auri- 
cles (au) are seensurrounded by the pericardial wall. 
Figure 20/ is in the region of the liver (/z), which 
has about the same position in relation to the auri- 
cles (az) 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 cesophagus (oe) are entirely distinct from 
each other; the former is a small, nearly circular 
hole, while the lumen of the latter is obliterated and 
