12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



pletelv fused, not merely in close contact. The same is true of the 

 tongue of cells between the oesophagus and trachea. Two or three 

 sections caudad to the one under discussion this tongue of cells loses 

 its connection with the trachea, and the latter structure is entirely 

 independent of the oesophagus. 



The solid condition of the oesophagus continues through about fifty 

 sections of this series, the horns of the crescent gradually shortening 

 until only the central part remains as the hollow cylinder seen in 

 figure 6f, oe, which is a section through plane 650 of figure 6a. 

 From about this point to its opening into the stomach the oesophagus 

 has essentially the same structure. Its epithelium is of the simple 

 columnar type, the cells being long, with generally basally located 

 nuclei. 



In the section under discussion the trachea, ta, is of about the 

 same size as the oesophagus, but its epithelium is thicker and consists 

 of two or three layers of cells. The trachea extends, as a separate and 

 distinct structure, through about one hundred and fifteen sections, 

 and tlien, at a point four or five sections caudad to the present sec- 

 tion, it divides suddenly into the two bronchial tubes. Each bronchus, 

 like the trachea, is lined with an epithelium of three or four layers 

 of cells ; but the epithelium is surrounded by a thin layer of much 

 condensed mesoblast. The bronchi continue caudad, with slightly 

 increasing caliber, through about fifty sections, when they suddenly 

 enlarge to form the lungs. As seen in figure 6a the lungs are irreg- 

 ularly conical in outline and lie on either side of the posterior end of 

 the oesophagus. 



Figure 6g is a section through the plane 750 of figure 6a. The 

 oesophagus, oe, is seen as a small, circular opening between two 

 much larger openings, the lungs, hi. The epithelium of the oesoph- 

 agus is the same here as in the more anterior regions described 

 above ; that of the lung rudiments is very variable in thickness, even 

 in different parts of the same section, being in some places com- 

 posed of a single layer of cuboidal or even flattened cells, in other 

 places consisting of four or five layers of cells (not well shown in 

 the figure). Surrounding the epithelium of the lung rudiments is 

 a thin layer of quite dense mesoblastic tissue. A fairly well defined 

 mesentery, ms, is now present in this region. 



Filling the greater part of the body cavity, below the oesophagus 

 and lung rudiments, is the liver, li; and ventrad to the liver the 

 section passes through a loop of the duodenum, d. 



The epithelium of the duodenum consists of four or five layers of 

 compactly arranged cells, near the center of an oval mass of fairly 

 dense mesoblast. In a lateral projection of this mass of mesoblast 



