184 GEO. S. HUNTINGTON 
borne out by the fact that in the aquatic mammals (pinnipedia, 
cetacea, sirenia) with bilateral pulmonary symmetry, the heart 
occupies frequently a striking median position, extending nearly 
equally into both sides of the thorax. It is of course impossible 
to decide whether this is the original condition favoring sym- 
metrical pulmonary development, or whether the initial devi- 
ation of the heart to the left has been corrected secondarily by 
an increase in the volume of the left lung and possibly by the 
operation of hydrostatic factors. 
Flint (21) regards the unpaired right eparterial bronchus ‘‘as 
the normal condition of mammalia, due to a phylogenetic pro- 
vision for the descent of the heart and great vessels through the 
suppression of the element of the left side.”’ 
Flint explains the mechanical factor which he considers opera- 
tive in preventing the development of a left eparterial bronchus 
as follows (1. c., p. 129): 
In the descent of the aortic arch and the Ductus arteriosus during 
embryonic life from a point above the origin of Lateral 1 to a point 
below, we have an explanation for the suppression of this element on 
the left side, for if this bronchus were formed, both aorta and the 
Botallian duct would be caught upon it and their descent prevented. 
Likewise the Vena pulmonalis appears in the midline and is carried to 
the left until it finally rests on the portion of the stem where a left 
Ventral 2 should develop. The usual suppression of these two elements, 
therefore, must be looked upon as a phylogenetic provision to allow 
for the descent of the great vessels on the one hand and the shifting 
of the Vena pulmonalis on the other. It is noteworthy that in those 
animals where these bronchi are formed on both sides, they are so 
situated as to offer no resistance to either of these features of the devel- 
opment of the great vessels. 
It is difficult to see just what Flint means in assuming archi- 
tectonic differences in the developmental groundplan of mam- 
malia possessing bilateral eparterial bronchi which favor sym- 
metrical bronchial unfolding in them, whereas it is prevented on 
the left side in the dominant mammalian type. It is also diffi- 
cult to bring his explanation of pulmonary asymmetry into line 
with the human instances of dextro-thoracic aortae without 
situs inversus, in which the aorta uses the right fourth arch and 
