288 The Alligator and Its Allies 
this section, is nearly as large in cross-section as all 
the rest of the embryo. As seen in such a section 
it is entirely detached from the body of the embryo, 
and in this particular case has about the shape of 
the human stomach. The mesoblastic portion 
of its wall (mes’) is of very irregular thickness; it 
forms a dense layer entirely around the outside, 
except for the pointed dorsal region, and is espe- 
cially thick along the ventral margin, where it is 
thrown into well marked folds, the heavy muscle 
columns. Lining the cavity of the heart is the 
membranous endothelium (en’), and between this 
and the dense outer wall just described is a loose 
reticular tissue with but few nuclei. 
As the series is followed toward the tail the 
sections diminish in size until, at a point about 
one third the embryo length from the posterior 
end, they are of scarcely one fourth the area of the 
sections through the region of the hindbrain. 
Figure 13d is about one hundred and twenty- 
five sections posterior to Figure 13c. Although not 
so small as the sections that follow it, this section 
is considerably smaller in area than the one last 
described. The amnion (a), which was not repre- 
sented in the last three figures, is very evident 
here. The spinal cord (sc) is considerably smaller 
here than in the preceding figure, while the noto- 
chord (nt) is not only relatively but actually larger 
than in the more anterior regions. Beneath the 
notochord is the aorta (ao), now a single large 
