NO. 2 LUNGS OF THE ALLIGATOR REESE 3 



extreme right of the figure is in the posterior region of the pharynx, 

 where the trachea begins to separate from the oesophagus. As may 

 be seen in cross sections, the pharynx is here of a crescentic outline, 

 convex dorsally, and hence is much smaller in cross section than it 

 seems in the figure under discussion. Projecting caudad and ventrad 

 from the horns of the crescent (figures 10, u and 12) are one or 

 more hollow, cylindrical bodies, perhaps the so-called epithelial 

 vestiges. The largest and most posterior of these, on the right side 

 is shown at e in figure 10. It is quite a conspicuous projection, 

 somewhat swollen near its distal end, lying laterad and somewhat 

 ventrad to the base of the trachea ; its mate of the left side is not 

 shown in figure 10. The other epithelial vestiges are smaller and 

 are not represented in this figure ; they may be discussed in a later 

 paper. The trachea, t, after separating from the oesophagus, ce, 

 extends caudad for some distance before it divides into the two 

 bronchi, b, b 1 . Its anterior region lies parallel to and fairly close to 

 the oesophagus, but at its point of divergence into the bronchi it 

 bends ventrad, so that the bronchi lie at a considerable distance 

 below the oesophagus. 



At this stage each endothelial lung rudiment consists of three 

 main lobes, I 1 to f, which project dorsad, on each side of the 

 oesophagus, at the region where the latter enlarges and passes 

 ventrad into the stomach, s. The mesoderm of the lungs is not 

 tabulated. 



Figures 11 to 18 represent transverse section through the respira- 

 tory tract in the planes shown on figure 10. 



Figure 11 passes through the posterior region of the pharynx, p, 

 where it still retains a somewhat crescentic outline, at the point of 

 origin of the trachea, or glottis, gl. The epithelium is here com- 

 paratively thin and the cavity of the pharynx comparatively 

 spacious. Around the glottis a condensation of mesoblast, la, 

 represents the beginning of the larynx. Dorsad and laterad to the 

 pharynx a cylindrical mass is the thymus, ty, while one of the 

 epithelial vestiges is shown at e. The spinal cord, notochord, etc., 

 may be recognized in this section, but need not be discussed here. 



Figure 12 is through the point of separation of the trachea and 

 oesophagus. The deep depression from the floor of the pharynx is 

 here widening to form a tube, t, the trachea. The pharynx, p, is 

 still of crescentic outline and its cavity is much reduced in extent. 

 The thymus anlage, ty, appears the same as in the preceding figure, 

 while the epithelial vestige, e, here shown is on the right instead of 

 the left side. 



