DEVELOPMENT 01- THE AMERICAN ALLIGATOR — REESE 55 



be seen, while ventral to it is noticed a faint condensation of meso- 

 blast (la), in the form of an inverted T, the anlage of the laryn- 

 geal structures. The ventral portion of the figure is made up of a 

 nearly circular, thin-walled cavity, the pericardium (pr). Most of 

 the pericardial cavity is occupied in this section by the thick-walled 

 ventricle (vn), above which is the bulbus (b) and the tip of the 

 auricle (an). The bulbus is nearly circular in outline, though its 

 cavity is very irregular. A few sections anterior to this, the opening 

 of the bulbus into the ventricle is seen. 



In figure 20g the section represented is only a short distance pos- 

 terior to the one represented by figure 20/. The mesoblastic struc- 

 tures in the neighborhood of the spinal cord (sc) and notochord 

 (nt) will be described in connection with the next figure, where 

 they are more clearly defined. The oesophagus (oe) — or posterior 

 end of the pharynx, whichever it may be called — is here a crescentic 

 slit, with its convex side upward; ventrally it opens by a narrow 

 glottis into the trachea (ta). The trachea is surrounded by the 

 same condensed area of mesoblast (la) that was mentioned in con- 

 nection with the preceding figure, but the condensation is here more 

 marked. From the bulbus (b) an aortic arch (ar) extends up- 

 ward for a short distance on the right side, while to the left of the 

 oesophagus an aortic arch (ar) is cut through the upper part of its 

 course. Ventral to the bulbus the ventricle (vn) and two auricles 

 (au) are seen surrounded by the pericardial wall. 



Figure 2oh is in the region of the liver (li), which has about the 

 same position in relation to the auricles (au) that was occupied by 

 the ventricle in the last figure. The auricles are connected with 

 each other by a wide passage. The trachea (ta) and the oesophagus 

 (oe) are entirely distinct from each other; the former is a small, 

 nearly circular hole, while the lumen of the latter is obliterated and 

 its walls form a solid, bow-shaped mass of cells. Since there is a 

 narrow space between this mass of cells and the surrounding meso- 

 blast, it might be thought that the lumen of the oesophagus had been 

 closed by the simple shrinkage of its walls ; higher magnification, 

 however, fails to show any sign of a collapsed lumen. It is doubtless 

 the problematic and temporary closure of the oesophagus that is 

 noticed in other forms. On each side of the oesophagus, in close 

 relation with the anterior cardinal vein (ac), is noticed a nerve 

 (en) cut through a ganglionic enlargement. When traced forward 

 these nerves are seen to arise from the region of the medulla, and 

 when followed caudad they are found to be distributed chiefly to the 

 tissues surrounding the newly formed bronchi; they are doubtless 

 the tenth cranial nerves. On the right side of the figure the close 



