Jolm Lewis Bremer 



71 



wide that many cells remain cuboidal, having no capillary over which 

 to spread a plate. This peculiar epithelium extends not only over the 

 inner surface of all the air-chambers, but also over the main bronchi as 

 far up as the beginning of the rings of cartilage. 



If we compare these lungs with those of some reptiles we find that 

 they are similar both in tlie arrangement of air-chambers opening into 

 a dilated main bronchus, and in the character of the epithelium, as is 

 shown in Fig. 7, drawn from the lung of Lacerta. Also in both the 

 opossum lungs and reptilian lungs there are bands of muscle fibres 

 running circularly around the central air-chamber or bronchus, whose 

 probable function is to contract the lung and force the air out during 

 expiration. Still further, in the reptilian lung the arrangement of 

 the bronchial branches is symmetrical, both ' right and left bronchus 

 being provided with one branch anterior to, and another posterior to 

 the pulmonary artery; and if we examine the drawing (Fig 4) and the 

 diagram (Fig. 10) made from the same opossum, we find that in the 

 new-born opossum also there is one bronchial branch in front of and 

 another behind the artery in both right and left lung. This was found 

 in five out of the six new-bom opossums; one was rendered useless for 

 serial work. In other words, the new-born opossum has an eparterial 

 bronchus on both right and left sides; that on the left is always the 

 smaller and slightly lower placed, and the air-chambers supplied by it 

 do not form ttie apex of the lung; still in spite of its small size and 

 relatively low position, it is distinctly above 

 the first ventral bronchus and behind the 

 artery and so corresponds to the eparterial 

 bronchus of the right lung, and may be con- 

 sidered as making the two lungs symmetrical 

 and reptilian in type, as no placental mam- 

 malian lungs are. This symmetry is marred 

 by the presence of a large cardiac lobe on the 

 right side, of which I can find no trace on the 

 left. Still, as regards general appearance, 

 character of the lining epithelium, and sym- 

 metry of bronchial branches (with this one 

 exception),, these lungs are, as Selenka says, 

 reptilian. 



Let us now trace the growth of this lung 

 further. On looking at Fig. 8, a section of 

 the lung of a 14 cm. opossum, we find the primary bronchi and their 

 early branches now provided with a thick coat, partly due to the 

 multiplication of the circular muscle fibres already mentioned, while 



Fig. 9. Photograph of cut 

 surface of lung of young adult 

 opossum. 



