PULMONARY EVOLUTION IN MAMMALIA 1 
marsupial genera. I obtained corresponding results, not en- 
countering a single left eparterial bronchus in my entire series of 
marsupial lungs, the details of which are included in a forth- 
coming publication. My material comprised 89 examples of 
18 marsupial genera, distributed among 46 species and including 
29 individuals of Didelphis assigned to 8 species, with 17 speci- 
mens of D. marsupialis (virginiana). I therefore believe that 
_ Bremer’s interpretation of his 12.5 mm. opossum embryo is at 
fault in disregarding the fact that in its typical course the left 
pulmonary artery of this form extends caudad in the interval 
between the rows of primary ventral and dorsal derivatives 
from the stembronchus in all embryonic stages, so that a dorsal 
bronchus is always placed behind, a ventral always in front 
of the artery at any given level of the lungstem. 
In the passage quoted above Bremer lays stress on the obser- 
vation that the left bronchial element interpreted by him in the 
12.5 mm. Didelphis embryo as an eparterial anlage arose dis- 
tinctly cranial to the derivation of the first ventral bronchus, as 
well as behind the artery. This high origin of a left first dorsal 
bronchus is very unusual. In all of my reconstructions of 
younger pouch embryos (3) and in the corrosions of older stages 
the bronchus has invariably arisen distinctly caudal to the level 
of V'. Its more cranial origin described by Bremer must, I 
believe, be considered a variant, as already suggested by Flint 
(21), which may have wrongly simulated a left eparterial anlage. 
As the concluding lines of the passage quoted above indicate, 
Bremer regards the reptilian lung as possessing a_ bilateral 
eparterial system and differentiating in this regard from the 
mammalian types. Both of these assumptions are incorrect. 
Many mammalian genera and species are now known to have 
typically a bilaterally symmetrical eparterial bronchial organi- 
zation. Others show in individuals in addition to the normal 
right eparterial bronchus a corresponding derivative from the 
left stembronchus as a variant acquisition. In both groups the 
eparterial components are distinct mammalian neomorph de- 
velopments and not reappearances of an archeal reptilian-char- 
acter. The assumption that the modern: reptiles possess a 
