8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



included in this figure. The main entodermal cavity on the right 

 side is very irregular in outline and is surrounded by several smaller 

 lobules which are not indicated in the reconstruction. 



Figure 24 represents a hasty reconstruction of the mesodermal 

 lung on the right side of an embryo of about seven centimeters 

 length. The lung, m, it will be seen, is without division into lobes 

 and is very deep dorso-ventrally, in proportion to its length. The 

 bronchus, b, enters it slightly caudad to its middle region. The 

 point of division of the trachea, t, into the two main bronchi is in 

 the plane of figure 25. The oesophagus, ce, is of large diameter, but 

 its apparently unusual size is partly due to its being laterally com- 

 pressed. 



Figure 25 represents a section through the body of this embryo 

 at the point of division of the trachea, t, into the two bronchi, line 

 25 in figure 24. The skeleton, aa, ce, r, is now w T ell outlined in 

 cartilage, and the lungs are approaching their adult condition. The 

 lung on the right side of the figure is somewhat larger than that on 

 the left and exhibits six or eight large endodermal cavities, I, l\ 

 etc., and numerous smaller ones. 



Just ventrad to the cesophagus is the trachea, t, with thick walls 

 in which several condensations indicate the formation of the tracheal 

 cartilages, as noted below. Surrounding the trachea are several 

 large pulmonary blood vessels, bv. 



Figure 26, through line 26 of figure 24, represents a section 

 through the region where the bronchi, b, enter the lungs. On the 

 right the bronchus is shown opening directly into the endodermal 

 diverticulum, f. Ventrad to the oesophagus, ce, and bronchi are 

 several large blood vessels, bv, five of which are grouped in a dense 

 mass of mesoblast. 



At this stage there has been but little change, histologically, from 

 what was noted in connection with figure 22, though the poor fixa- 

 tion of the material at hand makes it difficult to determine minute 

 details. In the cesophagus the epithelium is about as before, but 

 the surrounding, dense layer of mesoblast now exhibits a faint 

 division into a granular layer, next the epithelium and a more 

 fibrous layer outside of this ; the former may represent the sub- 

 mucosa ; the latter, the muscular layer, though this point has not 

 been worked out. In the trachea and bronchi the epithelium is 

 thinner than in the preceding stage and consists of only one or two 

 layers of cells. In the surrounding condensed layer of mesoblast 

 may be seen a number of small, darkly stained areas, ca; these, 

 under a higher magnification, are seen to consist of a closely-packed 



