Joseph Marshall Flint 65 
quently, is transferred from the systemic into the pulmonary system. 
Until this occurs, the pulmonary arteries do not even approximately cross 
the stem bronchi as Aeby suggests. Apparently, as we shall see later, he 
recognized this fact. Furthermore, my observations in older stages are 
in accord with the findings of Zumstein and Narath, who hold that, in 
the sense of Aeby, a true crossing on the part of the artery never exists. 
It seems to me important, therefore, for a logical conception of the 
architecture of the bronchial tree, that the terms “ eparterial and hypar- 
terial” or, at least, all that they imply should be abandoned. 
The pulmonary vein (Pl. I, Fig. 7 v) is seen at this stage with two 
small tributaries, one from the head and another from the caudal 
region running in the Mesocardium posterior. They are in connec- 
tion with other dilated capillaries which may be seen in the neigh- 
borhood of the lung sacs, but the latter have not become large enough as 
yet to form definite veins. The vascular apparatus of the lungs, then, 
at the period of the formation of the two lung sacs, consists in two small 
asymmetrical arteries passing down behind the primitive stem bronchi 
ending in an irregular capillary plexus about the dilated epithelial tubes 
from the ventral side of which run enlarged capillaries emptying into 
the pulmonary vein in the Mesocardium posterior. 
No particular change is observed in the next older embryo 8.5 mm. in 
the relationships of the arteries (Pl. I, Figs. 9, 10 ad. as.). With the 
lengthening of the stem bronchi, however, owing to the increased capil- 
lary field about the bronchi, the right and left pulmonary veins (PI. I, 
Fig. 9v) may be seen emptying into the common trunk which, in turn, 
now opens into the left auricle. In a pig 10 m. long, the pulmonary 
arteries maintain their general relationship to the trachea, the right 
passing ventral to Lateral 1 (Pl. I, Figs. 11, 12 ad). Continuing down- 
wards, they gradually extend behind the stem bronchi giving off branches 
to the irregular capillary plexus which surrounds the primitive tree, ele- 
ments of which may be seen, here and there, in well-prepared cross- 
sections of the lung. As a rule, the arteries lie on the dorsolateral 
aspect of the stem. At this stage, it is quite evident that the three first 
branches of the tree, practically in the same period of development, are 
growing without reference to the arteries as they are surrounded only by 
a capillary plexus derived from branches of the arteries and from which 
dilated capillaries empty into the veins. As they increase in size, the 
arteries and veins, which follow the various ramifications of the tree 
are formed from the capillary plexus according to the regular histo- 
mechanical laws. The two main tributaries of the vein (PI. I, Fig. 11 v) 
5 
