180 GEO. S. HUNTINGTON 
ventral branch of the pulmonary artery keeping pace. V* and 
A together now supply the new cranio-ventral pulmonary terri- 
tory, which tends to separate by fissuration as the primary ‘upper 
lobe’ from the remainder of the archeal lung-stem. The stem- 
bronchus, having yielded its first ventral primary bronchus V‘ 
to the newly acquired more mobile district, supplies with its 
remaining ventral (V"—-V*’) and with all its dorsal branches 
(D'—D'’) the definitive lung-stem, constituting the ‘lower lobe’ 
of the more highly developed mammalian lung. The left lung 
of the dominant mammalian type does not pass beyond this 
evolutionary stage and hence retains a more archeal character 
than the organ of the right side in the great majority of the mam- 
malia. In the special groups with bilateral eparterial organiza- 
tion (pinnipedia, cetaceans, camelidae), the left lung follows more 
or less closely in the wake of the right pulmonary extension. 
In figure 14, the anlage of Hp. destined to supplant A in the 
progress of further evolutionary advance, is shown schematically 
as an epithelial bud supplied by a special branch of the pulmonary 
artery. 
The ascending branch A of V' varies in relative size as compared 
with the parent stem in different mammalian types, as the nat- 
ural result of its phyletic development. 
1. It may be of smaller caliber than V‘, appearing then merely 
as a slightly enlarged side-branch. In this case the new pul- 
monary territory is located chiefly in the ventral area, the 
cranial extension remaining small. 
2. The first ventral bronchus may divide into two equal 
branches, A and V'. The primary upper lobe then extends 
equally craniad and ventrad. 
3. The first ventral bronchus arches almost directly from its 
origin craniad, (A) representing the main channel, while the con- 
tinuation of V' appears as its most caudal ventral derivative. 
Pulmonary extension is then chiefly craniad. 
The three conditions illustrate successive stages in the evo- 
lution of the cranial lung segment. Diagram 14 is based on the 
second. 
