270 The Alligator and Its Allies 
is found to open ventrally and medially into the 
anterior end of the heart. 
Figure 11d is about a dozen sections posterior 
to the preceding. The appearance of the over- 
hanging yolk (y), of the amnion (a), and of the 
notochord (nt) is about as in the more anterior 
section. The medullary canal (mc) is a straight, 
vertical slit, and the depression in the floor of the 
pharynx (ent) is much more shallow. The body 
cavity (bc) is much larger and extends across the 
mid-ventral line beneath the heart (ht), which is cut 
through its middle region. The heart may be 
traced through about twenty sections (one tenth 
the length of the embryo); its mesoblastic wall 
(mes’) is thin and irregular, and is lined by a dis- 
tinct endothelium (ex’) whose exact origin has not 
yet been worked out. 
Figure Ile is just back of the heart, and shows 
in its place the two vitelline veins (vv). The depres- 
sion in the floor of the enteron (en) is entirely 
distinct from the one that has been mentioned 
above, and is simply the posterior limit of the 
head-fold of the entoderm; the fifth section poste- 
rior to this shows where this depression opens ven- 
trally to the yolk sac. The other structures 
shown in the figure are not markedly different 
from what was seen in Figure 11d. 
Figure 11f is about one tenth the length of the 
embryo posterior to Figure 11e. The chief differ- 
ences here noticed are in the enteric and coelomic 
