PULMONARY EVOLUTION IN MAMMALIA 179 
lung-stem (figs. 14 and 15, V'-V7, D'-D’). The total number of 
the ventral and dorsal bronchi for the lung as a whole is thus in- 
creased through the supplemental elements contributed to the 
series by Ep and A. The secondary acquisition of the cranio- 
ventral pulmonary segments and their implantation upon the 
primitive lung-stem forms the key to the phyletic interpretation 
of the mammalian bronchial tree and of the pulmonary evolu- 
tion based thereon. These gradually acquired important mod- 
ifications of the primitive architectural type, and additions to it, 
and the resulting reorganizations are indicated schematically in 
the diagrams 12 to 15, showing a lateral profile view of a right 
lung. 
Figure 12 represents the archeal condition. The stembronchus 
gives off four larger ventro-lateral primary bronchi (V'-V"’) and 
an equal number of shorter dorsal branches (D'—-D'’). The 
pulmonary artery enters the cranial base of the lung-stem lateral 
to the extrapulmonary bronchus and turns dorso-caudad over V‘ 
to descend on the dorso-lateral aspect of the stembronchus be- 
tween the ventral and dorsal primary bronchi supplying a branch 
to each. 
Figure 13: The cranially directed derivatives of V! (stippled 
in diagram) enlarge, especially the first (A), foreshadowing the 
eventual cranio-ventral extension of the lung and the involve- 
ment of V‘ in the same. 
The dotted area (Ep) on the lateral surface of the extrapul- 
monary stembronchus, above and behind the artery, indicates 
the point of selection for the future development of the bronchial 
bud which will eventually lead to the establishment of the epar- 
terial bronchus. 
Figure 14: The cranial and ventral pulmonary segments have 
extended greatly. The former is built upon the first cranial de- 
rivative of V’, which, as its Ascending Branch (A), now passes 
well above the pulmonary hilus along the trachea, giving off 
ventral and dorsal side-bronchi and terminating in the distribu- 
tion to the apex of the lung. 
After supplying its ascending branch (A) to the cranial de- 
velopment, V! continues its original course ventrad, the first 
