PULMONARY EVOLUTION IN MAMMALIA 135 
side-branch of the first ventral derivative of the right stem- 
bronchus (Aeby’s first right ventral hyparterial bronchus). It 
then detaches itself from the parent stem and shifts or ‘mi- 
grates’ dorsad onto the right main stembronchus and joins a 
group of similar branches derived in the same way from the pri- 
mary ventral bronchi arising further down in the row from the 
stembronchus. It constitutes thus the most cranially located 
member (D') in the series of Aeby’s dorsal bronchi. As such D! 
comes to extend into the dorso-cranial lung segments, supplying 
its cranial pole, as Narath’s ‘apical’ bronchus. It possesses the 
faculty of continuing to wander further craniad along the stem- 
bronchus. When this migration has carried it above the level 
of the intersection of stembronchus and pulmonary artery it 
becomes, still as ‘apical’ bronchus, the equivalent of Aeby’s 
eparterial bronchus. It is able to extend its forward march, 
leaving the domain of the right stembronchus and gaining an 
origin from the right side of the tracheal bifurcation or from the 
lateral wall of the trachea above this level, as in the artiodactyl 
lung (Aeby’s type II”). In the left lung the same component 
apical’ bronchus) retains the primitive origin from the first 
ventral hyparterial bronchus, appearing as its ascending 
branch, does not emigrate and supplies the dorso-cranial seg- 
ment of the left upper lobe. 
If it follows the course taken by its homologue of the right 
side and wanders craniad onto the left stembronchus, Aeby’s 
bilateral eparterial type I* develops. If no shift occurs on either 
side the bilateral hyparterial tree is produced (Aeby’s type III). 
Other lateral derivatives of the primitive ventral branches, 
usually small and of minor importance, wander from their parent 
stem to the stembronchus and then appear as secondary branches 
of the same (accessory bronchi, Aeby’s ‘Nebenbronchien’), 
arising from the main canal in the intervals between the rows of 
principal ventral and dorsal bronchi. 
Ordinarily only one of these, derived originally from the first 
or second ventral bronchus, migrates to the stembronchus and 
attains a greater degree of development, supplying, as right car- 
diac (infracardiac) bronchus, the azygos or cardiac lobe of the 
