THE SYRINX OF GALLUS DOMESTICUS 181 



1. Mucous membrane 



In the early stages described above, the walls of the trachea and 

 the bronchi consist of an epithelial tube surrounded by a loose 

 mesenchyma. In the 68 hour stage the epitheUum of this tube 

 may be said to belong to the stratified variety. The boundaries 

 between the cells are not well defined, but two or three layers of 

 nuclei can be observed. The cells lining the lumen are mostly 

 columnar with their nuclei placed in the ends distal to the lumen. 

 The basal cells are much shorter and more irregular in shape. 

 Each bronchus presents a well defined lumen. 



No noticeable differences occur in the structure of the tube 

 in embryos of 72, 74, and 78 hours. In an 80 hour embryo, 

 however, the beginning of the basement membrane is seen as a 

 small differentiated line which extends around the outside of the 

 epithelial tube. Immediately below its origin from the pharynx 

 the tracheal tube has a lumen of oval outline. More caudally 

 it elongates from side to side until immediately above the bifurca- 

 tion it is little more than a transverse slit. At this level the 

 dorsal epithelial wall becomes thicker, while the ventral wall ap- 

 pears to fold inward and to come into contact with the dorsal 

 wall, thus dividing the one slit-like lumen into two. The lumina 

 of the two bronchi and the trachea have a smaller diameter at 

 80 hours than at any of the earlier stages. The epithelial walls 

 have not thickened at this stage, but the mass of mesenchyma 

 surrounding the digestive and respiratory tubes has condensed. 



In 128 and 140 hour embryos, the lumen of the tracheal tube 

 is so small as to seem almost obliterated in some sections. The 

 thickness of the dorsal wall is nearly double that of the ventral. 

 About 0.1 mm. cephalad to the bifurcation, the tracheal tube 

 becomes compressed dorso-ventrally, thus a slit similar to that 

 described above is produced. It possesses, however, a somewhat 

 larger lumen. 



The lumen of each bronchus possesses a greater transverse 

 diameter than that of the trachea, except in that region immedi- 

 ately above the bifurcation of the latter. The dorso-medial por- 

 tion of each bronchial wall is much thicker than any other part. 



JOURNAL OF MORPHOLOGY, VOL. 29, NO. 1 



