"172 GEO. S. HUNTINGTON 
B. Structural modifications of the primitive extrapulmonary 
conductory channels of the trachea and bronchi, in 
their reaction on the lung-stem 
We have in the preceding pages followed the main evolutionary 
lines leading up to the development of the most primitive type 
of the mammalian lung, consisting of the archeal lung-stem and 
its bronchial distribution (fig. 12). Additional extension of the 
respiratory area of the lung-stem is, as suggested above, possible 
to a limited extent by the increased development of its ventro- 
medial accessory bronchi leading to the production of one or 
more cardiac lobes. If the respiratory demand is further height- 
ened the cranial portions of the lung-stem are the first to be called 
upon, because pulmonary extension is only possible, for topo- 
graphical reasons, in the cranio-ventral direction. 
The first ventral bronchus is mobilized in the service of this 
new acquisition of respiratory territory. It enlarges and its 
first cranial side-branch assumes proportions which may bring 
it finally to equal or even to exceed the rest of the parent stem 
V:. As the Ascending Branch (A) of the first ventral bronchus 
it now controls an additional pulmonary district developed from 
the lung-stem by a modification of the primitive intrapulmonary 
bronchial plan (fig. 13). As this new area enlarges and extends 
craniad and ventrad it may acquire a greater independence by 
the development of a cleft in the mesoderm enveloping the lung, 
which then, as the anlage of the main interlobular incisure, di- 
vides the entire organ into a cranial and a caudal segment (fig. 14). 
The former is the primitive upper lobe (fig. 14, U), built upon the 
first ventral bronchus (V‘) and its ascending branch (A). The 
latter is formed by the balance of the original lung-stem, sup- 
plied by the stembronchus and its remaining primary ventral 
derivatives caudal to V! (V2-V4), all the dorsal bronchi (D'-D*‘), 
and the cardiac bronchus (C) (fig. 14, Lst). The result is a mam- 
malian lung whose bronchial tree is constructed in conformity 
with Aeby’s bilateral hyparterial organization (type III). 
Let us follow this first definite and typical mammalian bron- 
chial tree in its further evolutionary progress toward the develop- 
ment of a still greater peripheral respiratory extension. 
