30 
it differentiate 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 dorsal- 
wards 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 the ventral series, arteries pass around the lateral aspect 
of the stem bronchus beneath the root of the corresponding kateral 
bronchus to gain the outer aspect of the ventral bronchus along which 
it runs. The medial bronchi receive their supply from branches that 
originate from the main artery and pass around the dorsal aspect of 
the stem to run on the dorsal surface of the medial bronchi. As the 
right pulmonary artery runs ventralwards to Lateral 1 the artery to 
that bronchus develops on its ventral surface. 
In the younger stages, both the aortic arch and the Ductus arte- 
riosus lie well above the level of Lateral 1. As the embryo increases. 
in age, there is a gradual descent of the heart and with it, the great 
vessels. At 15 cm one observes the Ductus arteriosus at the level of 
Lateral 1; at 22 cm the aortic arch reaches this point while at birth 
both vessels lie below the bronchus. 
12. The pulmonary vein develops in pigs about 5 mm long as an 
ingrowth from the undivided portion of the Sinus venosus at the level 
of the pulmonary anlage. As the stem bronchi increase in size, right 
and left pulmonary veins develop from the capillary plexus which sur- 
round them. These, naturally, form on the ventral surface, with the 
bronchi between them and the arteries. Similarly, as the various prin- 
cipal bronchi are produced from the stem bronchus, veins are formed 
from the capillary plexus. The veins from the lateral bronchi lie below 
and ventralwards to the bronchi, those from the dorsal elements run 
along the medial aspect of the air passages to empty into pulmonary 
veins lying ventralwards to the stems. The veins from the ventral 
bronchi extend along the medial aspect of the bronchus and terminate 
directly into the pulmonary veins; those from the medial bronchi ex- 
tend along their ventral surface to empty in the larger veins accom- 
panying the stems. The vein from Lateral 1 runs along the ventral 
aspect of the bronchus somewhat ventralwards to the corresponding 
artery. This forms the single exception to the general alternation of 
artery, bronchus, and vein. As the embryo increases in age, the Vena 
pulmonalis, which originates near the mid-line, is gradually pushed to 
