Joseph Marshall Flint 61 
on the one hand and veins on the other, is due to their fortuitous location 
with reference to the primitive aorte and the venous ostia of the heart. 
Their growth in size bears a definite relationship to the velocity of the 
current in them, while their arterial or venous nature is determined by 
the character of that current, a high pressure pulsating column of blood 
giving rise to an artery, a low pressure constant current forming a vein. 
The nature of the current depends, naturally, mechanically upon its 
position on the arterial or venous side of the capillary plexus. In con- 
sidering the problems of angiogenesis in mammals, I called attention to 
the fact that Thoma’s principles do not explain all the facts of vascular 
development nor do they entirely accord with them. For example, the 
statement that a new growth of blood-vessels follows a rise of blood 
pressure in a capillary area must be considered only an hypothesis and 
not a demonstrated fact, for this would make the vascular system the 
stimulus for the new growth of cells, while it is much more probable 
that cells give the stimulus for the production of new capillaries. It is, 
of course, obvious that the principal factors that govern organic growth 
are resident in the cells rather than the blood-vessels as is indicated by 
their behavior in the embryo before the vascular system is laid down. 
In tracing the development of the intrinsic vascular system of the 
mammalian lung, it is also obvious that the vessels follow the same histo- 
mechanical and histogenetic principles which are active in forming the 
vascular systems of such organs as the Gl. submaxillaris and the Gl. 
suprarenalis. Different conditions in the chief cells of the lung, namely, 
those of the bronchial tree, and different relations of the arterial supply 
and the venous drainage, give rise to different relationships on the part 
of the arteries and veins in the pulmonary apparatus. In the suprarenal 
body, we have the formation of a blood vascular system with a well- 
marked capsular plexus from which the blood supply of the organ is 
derived, and in the submaxillary gland an organ, where the blood- 
vessels, as in the lungs, accompany the ducts. In the latter instance, 
however, the conditions are such as to give rise to a venous system where 
the blood is drained by Vene comites of the main arteries, while in the 
pulmonary circulation, a relationship exists in which the arteries and 
veins are separated from each other by means of the bronchial tubes. 
According to the studies of Bremer, 02, which have also been confirmed 
by Sakurai, 04, the pulmonary arteries in the pig appear to originate sym- 
metrically from the pulmonary arches like those of other mammals. At 
first they remain comparatively parallel and later (7-8 mm.) bend 
towards each other, sending out at the same time small branches which 
