18 ORIGIN OF THE PULMONARY VESSELS IN THE CHICK. 



COMMON PULMONARY VEIN AND ITS TRIBUTARIES. 



The first indication of the common pulmonary vein is a proliferation of angio- 

 blasts from the dorsal wall of the sinus venosus extending dorsally toward the gut 

 at the level of the lung-bud. This occurs in the chick at the stage of 20 somites 

 and is best seen in sagittal section (fig. 1). There is no venous opening into the 

 sinus at this time, indicating that the pulmonary vein is not established. This 

 primary proliferation of angioblasts soon shows a differentiation into a right and 

 left portion having distinct histological differences (figs. 2 to 5). The right two- 

 thirds forms a compact mass of endothelium of the Up of the left valve of the sinus 

 venosus (fig. 5), into which the mesothelium of the dorsal mesocardium extends. 

 On the left the angioblasts are larger and more loosely connected ; they extend dor- 

 sally to the surface of the gut and spread out in all directions over its ventral surface 

 in the plane of tissue between the endoderm of the gut and the dorsal mesocardium. 

 At the same time, angioblasts can be seen to differentiate from both sides of the dorsal 

 aorta and from the bulbus of the ventral aorta, until the whole ventral surface of the 

 gut is covered with a plexus of angioblasts which have not yet formed the capillaries. 

 It is possible that some of the angioblasts may differentiate in situ from mesoderm, 

 but I have not found any isolated clumps of these cells that would indicate that 

 this does actually occur. At this stage the thickness of the embryo precludes the 

 study of living cells in this region, which is necessary for direct proof of such a 

 process. The loosely meshed clump of angioblasts lying between the tip of the 

 lung-bud and the sums venosus on the left side (figs. 2 and 3) undergoes central 

 liquefaction and opens secondarily into the sinus venosus. This is the common 

 pulmonary vein, which at this stage is a blind pouch, as the plexus of angioblasts 

 covering the ventral surface of the gut is not patent but is merely a network of cells 

 connecting the common pulmonary vein with the ventral and dorsal aortae. This 

 plexus of angioblasts acquires a lumen and forms a capillary net, the splanchnic 

 plexus, which connects the lumen of the sinus venosus, through the common pul- 

 monary vein, to the dorsal and ventral aortse and cardinal veins. 



I am not prepared to state whether the lumen of this plexus of capillaries is an 

 extension of the lumen of the common pulmonary vein or of the ventral or dorsal 

 aortse, or whether, as in the case of the common pulmonary vein, it is produced by 

 central liquefaction. In the case of the pulmonic arches (sixth) there is definitely 

 an extension of the lumen through a cord of angioblasts, while the common pul- 

 monary vein is formed by central liquefaction. Both processes occur in early 

 blood-vessel formation and are probably dependent upon the hydrodynamics of 

 circulation in any given area. This would explain the different processes seen in the 

 case of the pulmonic arches in contrast to the common pulmonary vein. Fedorow 

 thought that the lumen of the sinus venosus extended into this endothelial pro- 

 liferation. In my sections the reverse seems to be true; the mass of endothelial 

 cells undergoes central liquefaction, forming a lumen that opens secondarily into 

 the sinus venosus. Figure 3 shows a stage in which central liquefaction has oc- 

 curred but there is no opening into the sinus. Figure 5 shows this process slightly 

 older and there is now an opening into the sinus at that point. 



