292 The Alligator and Its Allies 
becomes still thinner as it is followed posteriorly 
over the hindbrain (hb). Between the floors of 
the fore- and hindbrains, in the acute angle caused 
by the cranial flexure, is the anterior end of 
the notochord (nt), the only part of that struc- 
ture that lies in the plane of the section. Ventral 
and posterior to the notochord is a large cavity, 
the pharynx (ph), whose entoblastic lining can 
scarcely be distinguished under this magnification 
from the surrounding tissues. The stomodeal 
opening being as yet unformed, the pharynx is 
closed anteriorly; posteriorly also, owing to the 
plane of the section, the pharynx appears to be 
closed, since its connection with the yolk stalk is 
not shown. In the floor of the pharynx, almost 
under the reference line ph, a slight depression 
marks the position of the first gill cleft. In the 
mesoblast ventral to the pharynx and near the gill 
cleft just mentioned, a couple of irregular openings 
represent the anterior end of the bulbus arteriosus, 
posterior and ventral to which is the heart (hi), a 
large, irregular cavity. The dorsal aorta (ao) may 
be seen as an elongated opening in the mesoblast, 
extending in this section from the middle region of 
the pharynx to the posterior end of the figure where 
it is somewhat torn. Two of the eighteen or 
twenty pairs of mesoblastic somites possessed by 
this embryo are shown at the posterior end of the 
figure (s), where the plane of the section was far 
enough from the median line to cut them. 
