22 The Development of the Lungs 
In spite of its small size and low position, it is above the first ventral 
bronchus and behind the artery and thus, according to Bremer, makes 
the right and left side of the lung symetrical and reptilian in type as no 
placentalian lungs are. The complete symmetry of the young lung is 
marred by the presence of a cardiac lobe on the right side which is 
unrepresented in the left. Bremer states that the reptilian lung has the 
double eparterial bronchus and thus the lung of the opossum is reptilian 
in type. In its later phases, the lung is changed from the reptilian to 
the mammalian form by the loss of the left eparterial bronchus, the 
multiplication of its bronchi and the acquisition of a new type of air 
chamber. In a 14 cm. opossum no trace of the left eparterial bronchus 
remains but Bremer states he is unable to follow the degeneration of 
this element from lack of necessary stages. He believes, however, with 
Selenka, that in the opossum we have an epitome of the evolution of the 
reptilian lung to the mammalian lung by means of the changes noted 
above. 
The observations of Bremer at once recall the views of d’Hardiviller, 
who believes the left eparterial bronchus is always present in rabbits 
but subsequently degenerates. If this observation is confirmed it would 
seem to support d’Hardiviller’s contention, although Narath, it will be 
remembered, believes that d’Hardiviller was dealing with a variation. 
From Bremer’s statement that no other lungs of placentalia have the 
double eparterial system, it is apparent that he has overlooked Aeby’s 
description of the lungs of Phoca vitulina, Bradypus tridactylus, Didel- 
phinus delphis, Auchenia lama, Equus caballus, and Elephas Africanus, 
and some other nine species described by Narath and two species of 
Cebus by Huntington, making in all seventeen species where the condi- 
tion described by Bremer as exceptional in mammalia is permanent. 
We must also consider the possibility that Bremer is dealing with a 
dorsal bronchus placed abnormally high on the stem bronchus, especially 
as he states this bronchus did not supply the apex of the lung. The 
observations of Narath, 96, on Echidna aculeata are also suggestive in 
this connection as he states the relationships of the vessels, while young 
marsupialia are in the pouch suffer no further change either in the case 
of the arteries or the veins. Furthermore, Narath does not support Se- 
lenka, 87, with whom Bremer is, more or less, in accord in his observa- 
tions on the opossum lung as he finds the lung of Echidna develops like 
other mammalian lungs and is not differentiated from the developmental 
processes which are active in the production of the placentalian lung. 
He, therefore, does not approve of a comparison of the lung of marsupials 
with that of reptiles. Moreover, Hesser was unable to find an eparterial 
