Joseph Marshall Flint 69 
from the right pulmonary artery along the ventral surface of the bron- 
chus and then follows the main divisions of the bronchi. Both arteries 
pass down behind the stem, lying on their dorsolateral surface in the 
angle between the dorsal and lateral bronchi. From it, three series of 
vessels arise, namely, those to the lateral bronchi, which run on the dorso- 
superior surfaces ; those to the dorsal bronchi, which pass backwards from 
the stem artery on the laterosuperior aspect of the bronchus; and those 
to the ventral bronchi, which pass lateralwards around the stem bronchi 
to the lateral surfaces of the ventral group. Owing to the suppression 
of median bronchi on the tree of the 18.5 mm. embryo, the origin of the 
vessels to the median bronchi will be studied later in the corrosions of 
older embryos. 
The veins have two chief branches accompanying the stem bronchi 
on their ventromedial surfaces. They receive as tributaries, veins from 
the lateral bronchi, which run along their ventroinferior surfaces and 
join the stem vein by passing above the corresponding ventral elements. 
Branches from the dorsal series of bronchi run along the medial surface 
of the bronchi across the median aspect of the stem to empty into the 
veins on either side. A series of tributaries are also derived from the 
ventral bronchi, which, after a short course on the medial aspect of these 
bronchi, terminate abruptly in the stem veins. The vein from L. 1 hes 
ventral to the corresponding artery and empties into the vein of Lateral 
2 in the Vena pulmonalis. Thus we have the veins from the upper and 
middle lobe emptying together into the main Vena pulmonalis on the 
right, while the single vein from the upper left lobe joins the main trunk 
on the opposite side. Below, the veins accompanying the stem fuse just 
below the division of the trachea and empty at this point into the Vena 
pulmonalis. The moving of the veins towards the left, due up to this 
time to the asymmetry of the heart and the hyperdevelopment of Ventral 
2, is now somewhat exaggerated by the development of the inferior vena 
cava on the right side of the infracardiac lobe, which also presses this 
structure to the left and, accordingly, must be looked upon as a factor in 
increasing the asymmetrical position of the pulmonary veins. 
The next period of growth in the vascular system can be easily fol- 
lowed in specimens of the entire embryonic lung which, after fixation in 
some fluid like formalin or corrosive acetic to preserve the blood in the 
larger vessels, are subsequently cleared in oil of cloves or creosote. If 
the vessels are not too full both series are easily traced, but, in any case, 
the veins stand out distinctly. Owing to the complicated structure of 
the tree, however, the exact relationships of the arteries and veins to 
the bronchi are best seen in double corrosions in which, either the bronchi 
