ah 
the left by the increasing asymmetry of the heart, until it finally comes 
to lie over the area of the stem bronchus where a left Ventral 2 would 
have developed if such a bronchus were present. The hyperdevelop- 
ment of the Bronchus infracardiacus associated with the development 
of the Vena cava inferior to the right of that bronchus aids in pushing 
the Vena pulmonalis to the left. 
13. The asymmetry of the mammalian lung is associated with the 
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 pulmonalis appears in the mid line 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, there- 
fore, 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 ge- 
neral mesoderm about the head gut. As the stem bronchi appear, this is 
pushed out into the primitive coelom to form two irregular swellings, 
which indicate 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 
and 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 se- 
parating 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 
