64 The Development of the Lungs 
longer than in the preceding stage, through the further growth of the 
auricular septum now empties into the left auricle. 
In a pig 7.5 mm., the arteries (Pl. I, Figs. 7, 8 ad. as.) maintain the 
same relationship as those in the preceding stage, namely, the right 
lies more ventral than the left and also somewhat nearer the median 
line. Behind it, however, the evagination for the formation of Lateral 1 
has appeared. At this time, the artery consists simply of an endothelial 
wall supported by the surrounding mesoderm. Situated some distance 
from the trachea, it is absolutely impossible that such a structure should 
have a determining influence upon either the production or position of 
this or other branches of the bronchial tree. Furthermore, it is now well 
known that such vessels do not influence mechanically the growth of 
organs which they supply, but follow the developmental processes which 
are inaugurated in the chief cells of the organ itself acording to defi- 
nite histodynamic and histomechanical principles. 
By a glance at the schema which elucidates this point, we see how the 
two factors outlined above have worked to bring about the relationship of 
the artery to the primitive lung sacs. After its origin during the pro- 
duction of the primitive lung sacs, the lung anlage (Schema LA) ex- 
tends ventralwards. The Vena pulmonalis (Schema VP) in growing in 
from the auricle has established the venous outflow ventral to the an- 
lage, leaving the pulmonary arteries (Schema AP) to form on the 
dorsal side of the primitive stems. This relationship occurs, however, 
before there is the slightest indication of the presence of any of the main 
bronchi. Later as they appear, Lateral 1, the so-called “ eparterial 
bronchus” (Schema L. 1) develops behind the artery and Lateral 2 
(Schema L. 2) in front of it. Sometimes Lateral 1 is higher up, 
where it appears on the trachea, sometimes lower down where it forms 
on the stem, often where it forms on both sides, the left is lower than the 
right. The most important element in determining the position of 
Lateral 1 is the point at which the trachea separates into the two stems. 
As we have seen, when this is high, taking Lateral 2 on each side as the 
fixed topographical point, Lateral 1 is on the stem; when it is low, as 
in the pig, Lateral 1 forms on the trachea. 
It is also important to observe that the relationship between the 
Arteria pulmonalis and Lateral 2 is not “ eparterial” as Aeby suggests ; 
the artery in the embryo simply runs ventralwards to Lateral 1 and then 
passes gradually behind the stem. The “ eparterial and hyparterial 4 
topography of the bronchi is due to the descent of the heart in the later 
stages of embryonic life and to the degeneration of the Ductus arteriosus 
after birth when the entire circulation from the right ventricle, conse- 
