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bronchus, while the dorsal and ventral elements are formed somewhat 
higher up, often where the stem has regained its cylindrical form. 
Subsequent division of the branches may occur either by monopody or 
dichotomy. Often monopodial production of buds persists for one or two 
generations on the main bronchi, then the method becomes dichotomous, 
either equal or unequal in nature depending somewhat on the space in 
which the bronchi have to divide. In the case of equal division of the 
bud, however, one fork grows on to become the stem while the other re- 
mains as the side branch. The first division of the main bronchi may, 
it is well to note, be dichotomous as in the case of Lateral 1 and Lateral 
2. Thus in its growth, the mammalian lung recapitulates the history 
of the simpler lungs of lower animals. 
11. The pulmonary arteries in the pig arise from the pulmonary arches 
as Bremer has described. At first, they run parallel, then bend towards 
each other, sending out anastamoses, which yield finally a common trunk 
with two origins above and two arteries below. Later the upper part of 
the right artery degenerates and with it the right pulmonary arch. At 
5 mm. before the pulmonary arteries may be followed as far as the 
anlage of the lungs, the pulmonary vein may be seen as a slight ingrowth 
from the undivided portion of the Sinus venosus, passing through the 
Mesocardium posterior towards the pulmonary anlage. It forms almost 
in the medial plane. With this establishment of the venous outlet 
ventralwards to the anlage, the arteries, as the growth of the organ 
proceeds, are naturally developed from the capillary plexus on the dorsal 
side of the primitive bronchi. This fixes the arteries with reference to 
the stem bronchi before any of the side branches are produced. As the 
pulmonary anlage projects some distance ventralwards from the head 
gut, Lateral 1, the “eparterial” bronchus, develops above the artery, 
while Lateral 2 and the remainder of the principal branches originate 
below. Thus, the two regions of the tree have a different topography 
with reference to the pulmonary artery, but this vessel has no funda- 
mental influence on the structure of the two parts, nor does it differen- 
tiate the tree into two regions of different morphological significance as 
Aeby has maintained. 
The entire primitive tree is surrounded by a capillary plexus. As the 
bronchi grow, and produce new branches, arteries are developed from 
this plexus on the dorsal side of the tree as the artery lies dorsalwards 
and lateralwards to the stem. From this position, arteries to the lateral 
bronchi run out above and behind them. The branches to the dorsal 
bronchi pass dorsalwards along the lateral aspect of these elements. To 
