ORIGIN OF THE PULMONARY VESSELS IN THE CHICK. 23 



delicate capillary filament. I have injected such a stage, which is earlier than that 

 shown in figure 8, and in which there is a complete arch, in the form of an extremely 

 fine capillary, connecting the large dorsal and ventral pouches of the arch. This 

 is the earliest stage at which it is possible to inject the arch by this method. The 

 specimen was not used for illustration because other structures, due to faulty dis- 

 section, did not show clearly. 



As soon as the arch is complete it undergoes a rapid increase in size until it is 

 equal in importance to the other arches. Its position, connecting the ventral 

 aorta to the dorsal aorta, puts it in the direct line of arterial blood-flow. The 

 dynamics of increased pressure, rate of flow, and action of the heart are undoubtedly 

 responsible for this rapid increase in size. The pulmonary artery, lying in an in- 

 direct path connected with the venous circulation, has no such stimulus to growth 

 and remains a small, unimportant-looking vessel. The early connection of the 

 pulmonary artery with the ventral aorta, adjacent to the pulmonary arch, is soon 

 altered. The arch during its rapid growth actually carries the small artery along 

 with it, until in later stages the arterjr is seen to come off at the junction of the 

 venfcral and middle third of the arch. This early disproportion in size, together 

 with the relation of the artery to the arch at this stage, gave rise to the former 

 erroneous view that the pulmonary artery arises as a small branch from the arch. 

 In reality the two arise independently of each other, the artery actually antedat- 

 ing the arch. 



I wish to take advantage of this opportunit}' to acknowledge the generous 

 assistance and encouragement of Dr. F. R. Sabin, under whose supervision this 



work was done. 



SUMMARY. 



1. The first phase of the vascular system of the lung consists of masses of 

 solid angioblasts, rather than of a plexus of vessels, but although the origin of 

 the pulmonary system falls well within the period in which vasoformative cells 

 are seen to differentiate out of mesoderm, I have in my material no positive evidence 

 that the angioblasts giving rise to this system do actually differentiate in situ 

 from mesenchyme. No isolated clumps of these cells indicating such a process 

 are seen in my sections. A study of the cells of this region in a living blastoderm 

 is impracticable because of the dense intervening tissues. The angioblasts seen 

 are connected to other angioblasts, and the earliest cells are in continuity with and 

 lie near the endothelium of established vessels, and the zone between the gut and 

 the dorsal mesocardium is almost acellular before the spread of angioblasts into 

 that area. 



2. The first indication of the common pulmonary vein is a proliferation of 

 angioblastic cells from the dorsal endothelial wall of the sinus venosus at the level 

 of the developing lung-bud, seen in chicks of 20 somites. 



3. This mass of cells extends between the folds of the dorsal mesocardium 

 until the solid wall of the ventral surface of the gut is encountered. They then 

 grow out in all directions over the ventral surface of the gut, contributing to the 

 formation of the splanchnic plexus (20 to 30 somites). 



