4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



Figure 13 is through the region just caudad to the point of 

 separation of trachea, t, and oesophagus, ce. The former is here a 

 cylindrical tube with a lumen of considerable diameter, while the 

 latter is still crescentic in cross section and has no lumen at all. 

 This solid region of the oesophagus extends through a considerable 

 number of sections, and the fusion of the dorsal and ventral walls 

 is so complete that, even under high power, no indication of the 

 line of fusion is visible. At one side of the trachea is still seen the 

 epithelial vestige, e. 



Figure 14 represents a section through the middle region of the 

 trachea, t, which has here about the same external diameter as in 

 the preceding figure ; but, owing to the thicker walls, its lumen is 

 narrower than in the more anterior section. This section is just 

 caudad to the solid region of the oesophagus, ce, and shows the 

 reappearance of the lumen as a small, circular opening at each lateral 

 end of the now dumbbell-shaped oesophagus. A very small, irreg- 

 ular space is seen above and below the nearly solid oesophagus, as 

 though it had shrunk away from the surrounding tissue. 



Figure 15 shows a section passing through the embryo at the 

 point of division of the trachea into the two bronchi, b. At this 

 point the triangular, combined areas of the two bronchi are con- 

 siderably greater than that of the trachea. 



The mesoblast immediately surrounding the bronchi is consider- 

 ably denser than the general mesoblast of this region. A similar, 

 but less marked, condensation of the mesoblast is seen around the 

 trachea anterior to this region. The oesophagus, ce, is here cylin- 

 drical and exhibits a large lumen. 



Figure 16 represents a section through the bronchi, b, just 

 cephalad to the region where they expand to form the first lung 

 lobule, l 1 , figure 10. The bronchi here are much larger in diameter 

 than the oesophagus, ce, and each is surrounded by a narrow zone of 

 dense mesoblast. In cross section the bronchi are circular, and 

 their walls and that of the oesophagus are composed of a compact 

 epithelium of three or four layers of cells. At this point the 

 oesophagus and two bronchi lie at the angles of a nearly equilateral 

 triangle. 



Figure 17 passes through the second pulmonary lobe, f, figure 10. 

 That the section does not seem to quite fit the reconstruction is due 

 partly to the angle at which the reconstruction was drawn and 

 partly to a slight falling ventrad of the lungs and trachea, owing to 

 the softening of the wax. The oesophagus, ce, is here considerably 

 larger than in the preceding section, is compressed laterally, and lies 



