6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



are smaller than those of the anterior end. Some of the very small 

 secondary lobules are not shown in this figure. 



Figure 20 represents a transverse section through line 20 of figure 

 19 ; only the respiratory tract and immediately surrounding 

 structures are shown. In the center of the figure is seen the very 

 large oesophagus, ce, below which is the relatively narrow trachea, 

 t; the epithelium of the .former consists of only two or three layers 

 of cells ; that of the trachea consists of about four or five layers. 

 Surrounding both tubes is a fairly thick layer of condensed meso- 

 blast ; that surrounding the oesophageal epithelium consists of more 

 or less elongated or spindle-shaped cells ; that around the tracheal 

 epithelium consists of closely arranged spherical cells. Projecting 

 dorsad and laterad from the undifferentiated mesoblast that sur- 

 rounds the oesophagus and trachea are two rounded masses, the 

 mesodermal lung primordia, m. The mass on the left of the figure 

 is the one shown in figure 19 ; it is larger than the one on the right 

 and exhibits two lobules of the entodermal lung primordia, I, while 

 on the right only the anterior end of a single entodermal lobule is 

 shown. The entoderm of these lobules usually consists of a single 

 layer of rounded cells, but immediately surrounding this entoderm 

 is a thin, dense layer of somewhat flattened mesoderm cells. With 

 the low power, under which the figures are drawn, the entodermal 

 and mesodermal cells cannot be distinguished from each other. In 

 the mesoderm surrounding the layers just described may be seen 

 many small dark areas, c; these are the aula gen of the pulmonary 

 blood vessels. 



Figure 21 shows a section through the point of division of the 

 trachea into the two bronchi, b (line 21 of figure 19) ; each 

 bronchus is of as great diameter as the trachea of the preceding 

 figure. The oesophagus, ce, has the same general appearance as in 

 the preceding figure, but has increased somewhat in cross section. 

 The mesodermal lung primordia, m, are here much larger than in 

 the preceding section and that on the left is again larger than the 

 one on the right; neither shows any division into lobes. On the 

 left are seen three large and several smaller entodermal primordia, 

 l 1B , while on the right two large and one small entodermal cavities, 

 f, are seen. 



Figure 22 represents a section through line 22 of figure 19; it 

 passes through the point of emergence of the bronchi, b, into the 

 lungs. On the left the bronchus is seen opening into the most ven- 

 trally located entodermal cavity, l\ On the right the section is just 

 cephalad to the corresponding opening. The oesophagus, ce, is 



