THE EMBRYOLOGY OF THE OPOSSUM 185 



The respiratory bronchioles. Selenka (1887), who was the 

 first to describe the lung of the newborn opossum, said : 



The construction of the lung- is precocious in the sense that 

 alveoli capable of respiratory function are developed in an 

 extraordinarily short time; but it is retarded in the sense 

 that the multiplication of the alveoli is deferred to a later 

 time. Probably also hereditary phenomena are at work in 

 this strange developmental history of the opossum lung, 

 for the lung of the new-born has exactly the appearance of 

 a reptilian lung; and since the simple structure of the latter 

 is manifestly adequate for the marsupial young, it can be re- 

 tained during the early part of life. Not so with the 

 Placentalia, among which the embryo is retained much longer 

 in the uterus. Indeed, even with them the first anlage of the 

 lung is much like that of the reptile and the opossum, but these 

 simple alveoli do not function any longer as respiratory 

 organs, having now become the building loci for the definitive 

 infundibula and alveoli. 



Bremer ('04) confirmed Selenka 's description in general, 

 but suggested that it is incorrect to call these first functional 

 air chambers 'alveoli.' Histologically, they are really 

 peculiar, modified bronchioles, which function as respiratory 

 chambers for awhile, and then revert to typical bronchioles 

 after the true infundibula and alveoli have grown out from 

 them. The difference between these first 'alveoli' and the 

 true alveoli seems to have been recognized by Selenka in his 

 last line where he distinguishes between 'die einfachen 

 Alveolen' and 'die definitive!! Alveolen,' but Bremer 's termi- 

 nology is probably better. A rather more fundamental point 

 was made by Bremer in correcting Selenka 's impression that 

 the true alveoli are formed by the ingrowth of partitions 

 which subdivide the primitive respiratory chambers. Bremer 

 pointed out that the infundibula and true alveoli arise as di- 

 verticula from the respiratory bronchioles. 



One interesting point about these first respiratory chambers 

 is that they are lined with an easily demonstrable respiratory 

 epithelium. The illustration of this epithelium shown in 

 figure 58 is taken from Bremer. The true alveoli do not ap- 

 pear until quite late. Bremer found them in a 14-cm. opossum. 



