296 The Alligator and Its Allies 



A number of blood-vessels may be seen, the pair 

 lying nearest the notochord being the aortse {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 a rather 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 



