Joseph Marshall Flint 129 
asymmetrical development of the heart and its great vessels. 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 pul- 
monalis 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 development of the great vessels. 
14. The mesodermic portion of the lungs is derived from the general 
mesoderm about the head gut. As the bronchi appear, this is pushed 
out into the primitive coelom to form two irregular swellings, marking 
the anlagen of the two wings of the lungs. With the appearance of 
Lateral 1, on the right side, and Lateral 2, on each stem bronchus, 
swellings are observed on the two simple lungs just over these bronchi, 
giving rise to the simplest forms of the Lobus superior, Lobus medius, 
on the right side, and the Lobus superior on the left. The remainder 
of the mesoderm about the stem bronchus forms the anlage of the Lobus 
inferior on each side. With the formation of Ventral 2, the Bronchus 
infracardiacus, a swelling from the mesoderm forms over it which is the 
anlage of the Lobus infracardiacus. These swellings are first surrounded 
by shallow grooves, which with the rapid growth of the bronchi beneath, 
rapidly develop into deep fissures separating the various lobes from each 
other. With the further growth of these bronchi and the appearance of 
the series of bronchi on the stem, projections and fissures are formed over 
and between them and in the mesoderm. These are equivalent in all 
respects except in age and size, to the earlier fissures and swellings, but, 
under ordinary circumstances, never give rise to distinct lobes. This is 
due to the more rapid growth of the first bronchi, to the gradual in- 
creasing density of the mesoderm, and, lastly, to the environment of the 
several lobes of the lung. The right Lobus superior, containing Lateral 
1 does not belong to the dorsal region of. the lung as some authors hold, 
but to the lateral. The characters which make it appear as a dorsal 
segment are secondary and not primary. Likewise the portion of the left 
Lobus superior containing the apical bronchus belongs to the lateral 
H) 
