182 JAY ARTHUR MYERS 



This seems to be due to a greater number of cell layers rather 

 than to the elongation of the cells, since four distinct layers of 

 nuclei appear in the thickened part, while only 2 or 3 layers are 

 present in the thinner portions of the wall. The same condition 

 explains the thickening of the dorsal wall of the trachea. A very 

 distinct basement membrane is present in these stages. 



In 152 and 164 hour embryos the lumen of the trachea is still 

 very small. At a point 0.24 mm. above the bifurcation it begins 

 to enlarge, not only in the transverse diameter, but in the dorso- 

 ventral as well. A cross section of the lumen and walls of the 

 tube at this level presents a figure which is almost a perfect 

 square, and is much larger than that described in previous 

 stages. Of the walls composing the square, the dorsal is thickest, 

 th.e ventral thinnest, while the lateral are intermediate in this 

 respect. The dorsal wall possesses four layers of nuclei, the lat- 

 eral walls three, and the ventral wall two layers. 



Figure 12, from an embryo of 176 hours, shows the relative 

 thickness of the epithelium in the region of the tracheal bifurca- 

 tion. The epithelium is thickest just cephalad to the bifurca- 

 tion, where not less than three or four distinct layers of nuclei 

 are visible. At the level of, and shghtly caudad to the bifurca- 

 tion, the number of cells gradually diminishes, until only two 

 layers of nuclei are present. The epithelium projecting into the 

 tracheal lumen at the point of the bifurcation appears slightly 

 thicker than that lining the medial bronchial walls with which 

 it is continuous. At this stage, too, slight irregularities or evagi- 

 nations become noticeable in the inner walls of the epithelial 

 tube. These are due to encroachments upon the tube, from 

 without, of growing mesenchymal condensations which lie im- 

 mediately beneath and in direct contact with the basement 

 membrane of the epithehum. Only two of these evaginations 

 have developed to a stage worthy of mention. They are some- 

 what caudad to the tracheal bifurcation, one projecting into the 

 lumen from the lateral wall of each primary bronchus. Caudad 

 to the site of these evaginations the epithelium again thickens 

 and for some distance three layers of nuclei can be distinguished. 



