NO. 2 LUNGS OF THE ALLIGATOR — REESE 5 



immediately between the lungs on either side. The irregular out- 

 lines of the lungs and the variation in the thickness of their walls 

 are due, of course, to the plane of the section. The surrounding 

 zone of dense mesoblast is narrower than in the preceding section. 



Figure 18 passes through the third and most posterior lobe, I 3 , 

 figure 10, of the lung. At this point the ossophagus, oe, bends 

 sharply ventrad and rapidly enlarges to form the stomach, s. On 

 the right the lung is so cut as to exhibit two cavities, almost at 

 their point of union : a small, dorsal lobule, and a larger, ventral 

 one ; on the left the two lobules are cut caudad to their junction, 

 and the upper one is cut through its extreme caudal wall so that 

 its cavity does not appear in the section. The walls are of the same 

 character as in the preceding section, and the dense layer of sur- 

 rounding mesoblast is even narrower than in the more anterior 

 section. 



In this section a small mass of Wolffian tubules, Wt, is seen on 

 each side of the aorta and dorsal to the lungs ; while both in this 

 and in the preceding two sections the dorsal region of the liver is 

 seen in the lower part of the section, li. 



Figure 19 represents a reconstruction, on paper, of the endo- 

 dermal lung of the right side (together with the trachea and 

 oesophagus), of a later stage than the preceding. While the endo- 

 dermal lung here shows this comparatively complicated series of 

 lobules, the surrounding mesoderm (not shown in figure 19) is still 

 smooth in outline and free from lobules as might be expected from 

 the smooth, unlobulated condition of the adult lung. The extreme 

 right of the figure begins just anterior to the lung and shows the 

 oesophagus, ce, and trachea, t, the latter being now of much smaller 

 diameter than the former ; in the preceding stage they were of about 

 the same caliber. In relation to the size of the lungs the trachea is 

 now of much smaller diameter than before ; it is of considerably 

 greater length, though only its posterior part is shown in the figure. 

 The point of division into the two bronchi is at the plane of the line 

 21, but only one bronchus is shown in the figure. The point of 

 emergence of the bronchus into the lung is in the plane of line 22. 

 At the left of the figure the oesophagus, ce, is of much greater caliber 

 than at the more anterior end, but the difference is not so great as 

 is apparent in figure 19, because at the anterior end the long axis 

 of the cross section is horizontal, while at the posterior end it is 

 nearly vertical. Projecting cephalad are seen seven or eight larger 

 lobules of various sizes and shapes, some of which bear secondary 

 lobules ; projecting caudad are only five or six lobules, most of which 



