PULMONARY EVOLUTION IN MAMMALIA 189 
and as further confirmatory evidence in favor of regarding the 
mammalian eparterial unfolding as a relatively late and sec- 
ondary occurrence. If it has been included in the primary ances- 
tral plan of pulmonary organization (Reduction Theory) the 
arterial relations of the entire organ would have conformed to 
the same. On the other hand, after the archeal bronchial plan 
and the conforming arterial supply became established, the 
later modifications of both were adapted to the primitive con- 
ditions. Left-sided eparterial development is not altogether 
excluded, as shown by the extant normal groups of this type 
and by individual variants. Its accomplishment is only ren- 
dered more difficult than on the right side, and hence occurs less 
frequently. 
The unoccupied proximal segment of the left stembronchus 
is moreover additionally handicapped in its potential range of 
_secondary bronchial proliferation by the indirect relation to the 
left-sided aortic arch. The smaller and far less resistant azygos 
arch surmounting the root of the right lung offers in comparison 
‘only insignificant opposition to pulmonary extension in this re- 
gion, as shown by the instances in which the azygos is lodged in a 
split of the right upper lobe. 
Lastly, in judging all the factors possibly or probably active 
in the relative development of the two lungs, and responsible 
for the prevailing asymmetry, the persistence of the patent left 
ductus arteriosus of the mammal, and its relation to the intra- 
pulmonary circulation during the placental period, deserves con- 
sideration. I have already in a previous publication (25) 
touched on its possible hydrostatic significance in this connection. 
It will thus appear that both the developmental, structural 
and functional conditions of the lung and the intrathoracic rela- 
tions differ sufficiently on the two sides to account for the 
greater volume attained by the right lung in comparison with 
the left. All these conditions are, however, based on the funda- 
mental organization of the archeal lung-stem and its adaptation 
to the secondary cranio-ventral segments acquired during the . 
course of evolution. 
