PULMONARY EVOLUTION IN MAMMALIA 187 
branch the cranial pole of the left lung, corresponding to the 
area which on the right side receives the eparterial bronchus. 
The narrow interval between left vagus and left pulmonary 
artery may at times suffice for the passage of a bud derived in 
this situation from the left stembronchus. In such individuals 
the variants mentioned below (cf. p. 193) would occur The 
normal ascending branch from the first left ventral hyparterial 
bronchus, usually supplying this segment, would then be corre- 
spondingly reduced or default altogether. 
It is significant to note in this connection that in the rabbit, 
in which form both the adult variations above mentioned and 
their embryonic anlagen have been recorded (cf. p. 104), the 
cranio-ventral pulmonary extension, forming the upper pole of 
the left lung, is very much reduced compared with the right side. 
This is especially marked in some wild species, as in Lepus cam- 
pestris. The impulse to send an eparterial bud from the left 
stembronchus through the narrow vago-arterial interval, in 
spite of the reduced space available, would be fostered under 
such conditions by the smaller pulmonary area which it would 
be called upon to serve. 
I believe that the conditions just outlined furnish the adequate 
ontogenetic explanation of the prevalent asymmetry of the mam- 
malian bronchial tree, and that this is based primarily on the 
difference in the opportunity for cranio-ventral pulmonary 
extension afforded normally to the right and left lung respec- 
tively, in consequence of the cardiac and oesophageal rotation in 
opposite directions. The sinistral turn of the heart, and its 
effect on the initial direction imparted to the pulmonary arteries, 
depends upon the typical development of the left-sided mam- 
malian aorta and the retention of the dorsal segment of the left 
sixth aortic arch as the Ductus Botalli. This structure fixes the 
cardiac twist and turns the left pulmonary artery dorsad, while 
in contrast the right pulmonary artery, freed by the obliteration 
of the right sixth aortic arch in its dorsal portion, turns ventrad 
in response to the rotation. The enormous mechanical force 
exerted by the Botallian Duct on the position of the heart and 
on the adult arterial pattern is strikingly illustrated by compar- 
