2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



The structures surrounding those under discussion are shown, 

 but need no description. 



Figure 3 shows a section at the point of separation of the trachea, 

 t, from the pharynx or oesophagus, ce. 



Figure 4, six sections caudad to figure 3, shows the trachea 

 primordium, t, distinct from the oesophagus, ce. The latter has here 

 a circular outline, while the former is much wider from side to side. 



figure 5 is a few sections caudad to figure 4, and shows the 

 trachea at its point of division into the two bronchial primordia, b. 

 The oesophagus, ce, has the same appearance as in the preceding 

 figure. 



Figure 6 is a few sections caudad to figure 5, and represents the 

 two bronchi, b, as widely separated from the oesophagus and from 

 each other. As noted above, the right bronchus is of considerably 

 greater diameter than the left, but, as shown in figure 7, it does 

 not extend so far caudad. The epithelium is of the same character 

 as in the more anterior sections. 



The entodermal bronchial primordia are surrounded by a rounded 

 mass of pulmonary mesoblast, in, that bulges laterally into the 

 crescentic pleural coelom, pi, on each side. 



Figure 8 represents a reconstruction on paper of the pulmonary 

 tract of a slightly later stage. The pharynx, p, shows the same 

 deep groove in the post-pharyngeal region that was noted in the 

 preceding stage. It rather suddenly divides into the dorsal 

 oesophagus, ce, and the ventral trachea, t. The oesophagus gradually 

 enlarges as it passes caudad ; the trachea, which now extends 

 through a number of sections, diminishes slightly in caliber to its 

 point of division into the two bronchi, b, b 1 . Each bronchus is rather 

 irregular in shape, but gradually increases in diameter to form an 

 enlargement near its posterior end. In the embryo here repre- 

 sented the right bronchus, b, was of greater caliber but of less 

 length than the left, b 1 . At the point of separation from the trachea 

 the bronchi lie at a considerable distance ventrad to the oesophagus, 

 but as they pass caudad they gradually approach the horizontal 

 plane of the oesophagus until they lie practically on each side of it. 

 The histological structures are about the same as in the preceding 

 stage so that no sections of this stage need be figured. 



Figure 9 represents, in outline, an embryo of the next stage to 

 be described. The appendages are here well developed and the 

 face is beginning to assume form. 



Figure 10 is a camera sketch of a wax reconstruction of the 

 entodermal respiratory tract of the embryo shown in figure 9. The 



