Joseph Marshall Flint 67 
of the tree. On the ventral surface of Lateral 2, veins originate, which 
empty into the right and left pulmonary veins, while medialward and 
above Ventral 2 lies the vein of that bronchus which joins the right pul- 
monary just below the tributary from Lateral 2. In this stage, either 
owing to the hyperdevelopment of Ventral 2, or the increasing asym- 
metry of the heart, or both, the pulmonary veins are shifted somewhat to 
the left, causing them to he somewhat beyond the median line. At the 
same time, the veins in these young stages are frequently reduplicated 
as the final channels are not always definitely selected. In order to show 
the different branches of the tree without extra illustrations, in this and 
the succeeding reconstructions, the pulmonary vein has been kept in the 
median line, and only the chief channels are shown in the case of redu- 
plication, which is a frequent occurrence. 
In a pig 15 mm. long, the pulmonary artery (Pl. I, Fig. 17) on the 
right side still has a more ventral and medial position than that on the 
left, a fixed relationship from embryos 12 mm. in length as the arteries 
both rise from a common trunk originating from the left pulmonary arch. 
Just below the point of origin of Lateral 1, the artery to that trunk is 
observed (Pl. I, Fig. 17), which passes up and divides with it into its 
ventrosuperior and dorsoinferior branches. The two pulmonary arteries 
bending dorsalwards pass back of the right and left bronchi, giving off 
the branches to the second lateral bronchi, which lie on their dorsal and 
superior surfaces. On the right side, the artery to the second ventral 
bronchus (PI. II, Fig. 17) has increased in length with the growth of that 
branch, while arteries to the second dorsal bronchi (Pl. II, Fig. 18) 
are observed passing along their lateral walls. From this point, the 
pulmonary arteries continue on in the angle between the dorsal and 
lateral bronchi, giving off branches to the third and fourth lateral ele- 
ments (Pl. II, Fig. 18) on each side which lie above and behind them. 
From the capillary plexus around the termination of the right and left 
stem bronchi, the beginnings of the pulmonary veins (PI. II, Fig. 17) 
are seen as in the preceding stage. From the fourth lateral and third 
lateral branches on either side, veins are formed which lie below and in 
front of these bronchi and pass in front of the stem bronchi to empty 
into the pulmonary veins, which lie upon their median and ventral as- 
pects. The vein from the second ventral bronchus (PI. II, Fig. 17), as 
in the younger stage, is placed medially to it and empties into the right 
pulmonary at the base of the third lateral bronchus. The veins from 
the second laterals have increased considerably in length, and lie on the 
ventral aspect of these divisions, while the Vena pulmonalis, formed by 
the confluence of the two right and left veins, lies ventral to the trachea 
