PULMONARY EVOLUTION IN MAMMALIA 185 
the order of the primary branches is left innominate, right caro- 
tid and right subclavian, while the right lung continues the nor- 
mal development of its eparterial element. Further the numer- 
ous instances encountered in the mammalian series and briefly 
referred to below (cf. p. 193), in which a left eparterial element 
develops as a variant in addition to the normal right branch of 
the same denomination, speak against Flint’s hypothesis. 
The concept of a left sided aorta and Botallian duct being 
‘caught upon’ a bronchus of this type and being prevented from 
descending thereby does not appear to me well based. The car- 
diac descent does not carry the aortic arches into a secondary 
relation with an already formed bronchial tree. When bron- 
chial budding begins the arches are already in their future and 
definite position in relation to the developing lung. Any sub- 
sequent topographical changes heart and lung undergo together, 
during the progress of their concurrent development. 
On the other hand a careful examination reveals intrinsic 
factors favoring the development of the dominant mammalian 
type of pulmonary asymmetry with hyperplasy of the right or- 
gan. These are based on the thoracic visceral and vascular 
relations of the lung and its adaptations to the same. 
A detailed study (27) of the topographical relations of the 
developing mammalian lung and of the pulmonary artery in the 
critical stages accounts very definitely for the prevalent right- 
sided eparterial development, and at the same time shows the 
possibility of the occasional derivation of a homologous eparterial 
bud from the left stembronchus. The topographical conditions, 
while favoring the development of the eparterial component in 
the right bronchial tree, are all against a corresponding eparterial 
derivative from the left stembronchus. Nevertheless the con- 
dition occasionally occurs as an individual variant in the em- 
bryo and adult of mammalian forms which possess normally 
the dominant asymmetrical bronchial type, as shown by the 
instances cited above (Homo, Echidna, Lepus, Tragulus, Ere- 
thizon, Sphingurus, Erinaceus). 
The evolutionary opportunity for the normal development of 
the bilateral eparterial type in certain limited mammalian groups 
