70 



On the Lung of the Opossum 



new buds, all of which are like the parent stem, pushing into the sur- 

 rounding mesenehyma. Just before birth a new kind of bud, with a 

 different system of division, is developed from the end of the last set 

 of branches, and these form the infundibula and alveoli, the true 

 breathing portion of the lung. In the young opossum, which is trans- 

 ferred to the pouch when only about 10 mm. long, breathing must be 

 carried on at the same time as the growth and branching of the bron- 

 chial tree; so instead of the usual short buds of cuboidal epithelium, 

 as found in placentalia, in the young opossum large chambers are found, 

 representing the narrow tubes, but lined with peculiar epithelium so that 



they may serve as respiratory 

 organs. These chambers are 

 not, however, to be considered 

 alveoli, but bronclii and bron- 

 chioles : and they retain their 

 power of sending off new buds 

 or branches, which may be seen 

 in the model and in the sections 

 as hornlike processes, hollow and 

 slender at first (soon widening 

 into large chambers), pushing 

 into the surrounding mesen- 

 ehyma, and giving evidence that 

 Selenka's idea of division into 

 a bronchial tree by means of 

 newly forming partition walls 

 is wrong. The lung of the new- 

 born opossum is composed of a 

 simple system of branching 

 bronchi and bronchioles, dilated 



Fig. 5. Opossum, 13.5 mm. Series 618, No. 303. i linpd with modified enithe- 



Ep, epithelium Uning air-chamber; mits, muscle; anci imeu W lUl luuuimu tpitiic 



'"'^^T'^^i^^'r^%:'i^iS^^f limn to allow for l)reathing, 



P^Km^. Lacerta. Series 604, No. 325. but retaining their pOWCr of 



Fig. 8. Opossum, 14 cm. Trans.section of lung. x'.„4-i,„t, ornwth 

 Pr, new bronchial branches ; i»/, infundibula. luriner growui. 



On examining the epithelium 

 lining these air-chambers, we find what seems to me a transitional stage 

 between cuboidal and "breathing epithelium" (see Figs. 5 and 6). 

 Directly over the capillaries, three of which are cut across in Fig. 6, the 

 cells have become squamous, with a thin plate and the nucleus lying 

 between the blood vessels : but the plates are not to be compared in thin- 

 ness with those of the human lung, for instance (in one cell in Fig. 6 

 the nucleus lies in the plate) ; and the meshes of the capillaries are so 



