62 The Development of the Lungs 
finally fuse into transverse anastomoses which yield ultimately a common 
trunk with two origins above and two main pulmonary arteries below. 
Bremer suggests that the bending of the arteries towards each other may 
be caused by the growth of the right and left auricles. This state of 
affairs occurs in the pig 11 mm. long. Later, the upper part of the 
right artery degenerates, and, with it, finally the right pulmonary arch. 
Thus we have the next stage where both arteries arise as a common trunk 
from the left pulmonary arch. 
In the earlier pig’s embryo (5 mm.), the arteries arising from the 
pulmonary arches on each side may be followed caudalwards a short 
distance from their origin on the arches, but only in particularly good 
specimens, as they are soon lost in the irregular capillary plexus sur- 
rounding the head gut to which, in their course, they give off frequent 
branches. At the same period, it is also possible to note the ingrowth of 
the pulmonary vein from the yet undivided portion of the auricle. It 
may be seen in a few sections running dorsalwards in the Mesocardium 
posterior towards the pulmonary anlage, which is, as yet, only partially 
separated from the cesophagus. It is asymmetrical as it les shghtly to 
the left of the medial plane. Its branches connect with the capillary 
plexus about the head gut and pulmonary anlage, establishing a venous 
outflow on the ventral side of the respiratory apparatus. Concerning 
the early appearance of the Vena pulmonalis in the pig, my observa- 
tions are in accord with those of Narath on the rabbit for in these ani- 
mals, the Vena pulmonalis is apparently evident at a much earlier stage 
than His, 87, or Schmidt, 70, were able to observe it in man. 
At 6 mm. after the formation of the primitive lung sacs is well under 
way, the pulmonary arteries may be seen (PI. I, Figs. 5, 6 ad. as.) run- 
ning in approximately parallel courses until they diverge and are lost 
behind the right and left bronchi in the capillary plexus about the primi- 
tive lung sacs. Their course, however, on the two sides is different owing 
to the horizontal position of the left stem bronchus, the artery on that 
side (Pl. I, Fig. 5as) is forced to turn dorsalwards in order to pass 
behind the left sac sooner than the right pulmonary artery, which main- 
tains its more ventral course and, finally, at a lower level descends behind 
the right stem bronchus. 
The factors which determine the course of the pulmonary artery in 
passing behind the lung sacs are, first of all, the ventral position of the 
venous outlet into the Sinus venosus, leaving the arteries to develop 
from behind. That is to say, with the increasing size of the right and 
left stem bronchi and the consequent enlargement of the capillary plexus 
