NO. 2 LUNGS OF THE ALLIGATOR REESE 7 



somewhat larger in cross section than in the preceding- figure, and 

 the lungs have a greater area than in any other section ; they extend 

 from the level of the lower side of the oesophagus to nearly the 

 level of the ventral side of the notochord, n. Each lung shows 

 several small and two large entodermal cavities ; the mesoderm is 

 still without lobules, and is continuous mesially and ventrally with 

 the mesoderm that surrounds the oesophagus. The entire lung of 

 the right side is not shown in the figure ; it has the same general 

 outline as that of the left side. 



The wall of the oesophagus at this stage is seen, under higher 

 power, to consist of a thin lining epithelium and a dense layer of 

 surrounding mesoblast, the latter being, on an average, about four 

 times as thick as the former. The epithelium, ep, consists of one or 

 two layers of cubical cells ; the basal cells are usually the larger. 

 The surrounding mesoblastic layer, ml, consists of flattened cells 

 that are evidently turning into fibers, and lie with their long axes 

 parallel to the epithelial layer. 



The walls of the trachea and bronchi consist of the same two 

 layers, but the epithelium is much thicker than that of the oesoph- 

 agus, and consists of. three or four layers of cells, the basal cells 

 being again much larger than those nearer the lumen. 



The mesoblastic layer is of about the same thickness as that of 

 the oesophagus but consists of closely packed spherical cells instead 

 of the elongated cells seen in the former place. 



The lung cavities are lined by a thin epithelium which consists, 

 in most places, of a single layer of cuboidal cells ; in many places, 

 however, are seen cresentic thickenings which consist in their 

 thickest part, of four or five layers of cells. These crescents are 

 usually seen in the bottoms of the smaller diverticula from the main 

 lung cavities. Surrounding the epithelial lining is a thin, indistinct 

 layer of slightly condensed mesoblast, scarcely discernible under 

 the low power used in drawing this series of figures. 



Figure 23 shows the conditions that are to be seen at a plane 

 about half way between the openings of the bronchi and p the pos- 

 terior ends of the lungs, line 23, figure 19. The oesophagus, ce, is 

 very large in this region and might, perhaps, be called the stomach, 

 since, ventrad to it, is seen the liver, li. In the upper part of the 

 figure are seen two masses of Wolffian tubules, Wt, attached to the 

 mesentery on either side of the dorsal aorta, a. 



The two lungs are of somewhat smaller area than in the preceding 

 section and that on the right shows an indication of a division into 

 a mesodermal lobe at f. Part of the lung on the right side is not 



