296 The Alligator and Its Allies 
A number of blood-vessels may be seen, the pair 
lying nearest the notochord being the aorte (ao), 
while the two other pairs, on either side of the 
fore- and hindbrains, are the anterior cardinals (ac). 
The first aortic arches are shown at ar. On the 
left the section passes through the exterior opening 
of the first gill cleft (g’), so that the mandibular 
fold (md) on that side is a distinct circular struc- 
ture, made of a dense mass of mesoderm surrounded 
by arather thick ectoderm. ‘The mesoderm of this 
fold is especially dense near the center, probably 
the beginning of the visceral bar. Near the center 
is also seen the aortic arch that has already been 
mentioned. On the right the section does not 
pass through the external opening of the first gill 
cleft (g’) so that the tissue of the mandibular fold is 
continuous with the rest of the head. It is of 
course the slight obliquity of the section that 
causes the pharynx (ph) to be completely enclosed 
on the right, while on the left it is open to the 
exterior both through the gill cleft and between 
the mandibular fold and the tip of the head. The 
superficial ectoderm shown here as a heavy black 
line varies considerably in thickness, being thickest 
in the region of the nasal pit already mentioned 
and thinnest over the roof of the hindbrain. The 
amnion (a) in this, as in the other sections of the 
series, has the appearance of a thin, very irregular 
line. 
Figure 15c is posterior to the region affected by 
