278 The Alligator and Its Allies 
comparatively thin, while the floor is thickened 
and depressed to form a deep, wide pit, traceable 
through six or eight sections. This pit may be the 
thyroid gland already noticed in the preceding 
stage. Below the main cavity of the pharynx and 
close to each side of the thyroid rudiment just men- 
tioned is a large blood-vessel (iv). These two vessels 
when traced posteriorly are found to be continuous 
with the anterior end of the heart, and hence 
may be called the truncus. They were noticed in 
Figure 11c, bv. The ectoderm surrounding the 
lower side of the embryo was so thin and indistinct 
that it could scarcely be distinguished from the 
mesoderm of that region. The amnion (a) is still 
a continuous envelope entirely surrounding the 
embryo. 
Figure 12d, about twenty sections posterior to 
Figure 12c, is in the posterior heart region. The 
spinal cord (sc), as might be expected, is smaller 
than in the more anterior region, but is otherwise 
not markedly different from what was there seen. 
The notochord (nt) also has the same appearance 
as before. The enteron (ent) shows of course in 
this region no gill clefts; it is a small, irregular 
cavity with thicker walls than in the figure just 
described. The ventro-lateral depression is entirely 
distinct from the depression that was called the 
thyroid rudiment in the preceding figure. Dorsal 
to the enteron are the two dorsal aorte (ao), 
now smaller and more ventral to the notochord 
