PULMONARY EVOLUTION IN MAMMALIA 181 
When fully developed the component (A) functions as a 
stembronchus for the cranial lobe. Its ventral and dorsal 
branches fall into serial line with the corresponding derivatives 
of the main stembronchus within the lung-stem, increasing the 
total number of these elements for the entire lung. 
Diagram 14 indicates three additional ventral bronchi and the 
same number of dorsal derivatives in the zone for which. (A) 
plays the part of the stembronchus (V'-*, D3) 
Regarding the organization of the lung in its new form the 
archeal V! becomes V4, D! is rated as D4, and so on down the line 
until the total number in each of the rows is raised from four to 
seven. In this and the succeeding diagram 15, the archeal des- 
ignation of each primary bronchus is indicated by the Roman 
numerals, its more recently acquired value in the series by the 
Arabic figures. 
Figure 15. In this stage the eparterial bud of figure 14 has 
expanded into the fully developed bronchus of that designation, 
supplying the ‘upper lobe’ in its later restricted sense. Its ven- 
tral and dorsal branches (V1, D*-*) are further extended and 
more directly aligned with those of the main stembronchus, and 
the eparterial arteries are correspondingly developed. Branch 
A of V* (formerly V’) has receded to the position of the normal 
first cranial side-branch from its parent stem. 
The district supplied by Ep may be peripherally demarked, 
by the development of the secondary interlobar incisure, as the 
‘upper lobe.’ In this case V4 (V") supplies the separate ventrally 
placed ‘middle lobe.’ In the other cases Ep and V4 (V") are 
still enclosed within the limits of a single primary ‘upper lobe.’ 
The individual areas supplied by single side-branches of Ep, as, 
e.g., V1, D!, may be outlined superficially by secondary fissures 
or grooves, forming apical clefts or incisures. 
Summary of eparterial development in the mammalia 
The phyletically evolved division of the archeal single lung 
into two distinct segments affects the intrapulmonary architec- 
tonics radically. The lung-stem remains supplied by the stem- 
