72 



On the Lung of the Opossnm 



pul. 

 :-'-'-'art. 



the lining epithelium has reverted to the cuboidal type or even 

 become' cylindrical. We have found now the reason for the peculiar 

 epithelium seen when these passages were breathing spaces; it was a 

 compromise allowing enough oxygenation of the blood for an animal 

 whose existence is passed in the mother's pouch, and yet not far enough 

 removed from the cuboidal type to make it hard to revert to it. 



The bronchial tree has become quite complicated and at the surface 

 of the lung may be seen in cross-section new hornlike processes (pr.) 

 representing newly formed branches. But with these are terminal pieces 

 of much larger size, often with triple branching, seen chiefly on the 

 surfaces of the lung where growth has nearly ceased. They represent 

 a new element in the opossum lung, but one found in placental lungs 

 just before birth, namely the infundibular portion of the lung (inf.). 

 They may be seen forming a cortex in Fig. 9, a photograph of the 

 cut surface of the lung of a young adult opossum; but with age they 

 become inconspicuous because more evenly distributed. They mark 

 the end of the stage of rapid growth, for from these infundibula no 



branches, unless we count 



the alveoli, are given off; 



and so they are absent from 



all activity grov/ing portions 



(such as the borders in Fig. 



8), their places being taken ■^ep ^ 



by the hornlike processes, 



which are capable of fur- 

 ther growth. The lung has 



changed from a reptilian 

 „ ,„ „ . to a mammalian type part- 



FiG. 10. Diagram. '' ^ ^ 



lung of opossum of ia.5 w j^y the multiplication of 



mm. seen from in front. J J ^^ 



Pul. art, pulmonary ^he bronchial branches, but 



artery; card, cardiac ^ ' 



i^n'cf^i'htTpI^t^^eV^fil Chiefly by the addition of a ^,,.1, ?ia.ram: Lung^of 



bronchi. VI, m-st ven- ^ew class of air chambers, op^fJi^^ °* ^* ^°^- ^^'^^ ^'^^^ 



tral bronchial branch. ^ inrioui. 



growing from the ends of 

 the bronchioles, but differing from them m that the spaces are dilated 

 instead of horn-shaped or tapering, and are lined with true "breathing 

 epithelium," with narrow-meshed blood vessels and very thin plates. 



The opossum lung changes from reptilian to mammalian also in the 

 loss of the left eparterial bronchus. How this comes about I am unable 

 to state for the lack of the necessary stages, for already in the opossum 

 of 14 cm. the change is complete, as can be seen in Fig. 11, a diagram 

 of the bronchial branches and the arteries of an opossum of that size, 

 where no trace of a left eparterial bronchus remains. 



