24 ORIGIN OF THE PULMONARY VESSELS IN THE CHICK. 



4. The core of angioblasts between the primitive gut and the sinus venosus 

 becomes differentiated into two parts. The right two-thirds is a compact mass of 

 endothelium forming the left valve of the sinus venosus; the left third undergoes 

 central liquefaction and opens into the lumen of the sinus venosus. This is the 

 common pulmonary vein in the form of a blind pouch connecting the sinus venosus 

 with the angioblasts on the surface of the gut (24 somites) . 



5. The angioblasts on the ventral surface of the gut in the region of the develop- 

 ing lung-bud acquire a lumen and form the splanchnic plexus (30 to 35 somites). 

 The four tributaries of the pulmonary vein are surviving vessels in this plexus of 

 capillaries. The veins from the right and left lobes persist and develop with the 

 lungs. The post-caval connections disappear at about 90 hours of incubation. 

 The cranial tributary loses its arterial connections and disappears at about 100 

 hours of incubation. 



6. The pulmonary arteries are persisting longitudinal vessels in the cephalic 

 portion of the splanchnic plexus of capillaries. The angioblasts giving rise to these 

 capillaries begin as a caudal extension of angioblasts from the endothelium of the 

 ventral aorta. 



7. The pulmonary arches (sixth) arise in the cephalic portion of the splanchnic 

 plexus at the stage of 35 somites. The angioblastic precursors of the arches are 

 derived from two sources, the dorsal rudiment from the junction of the dorsal 

 aorta and fourth aortic arch, the ventral rudiment from the ventral aorta. 



8. The pulmonary arches and arteries arise in the same plexus of capillaries, 

 but independently of each other. The arteries are patent before the arches are 

 complete. As a result of unequal rates of growth, the arch increases more rapidly 

 in size than the artery and includes the mouth of the artery within its wall. This 

 relation and early disproportion between the arteries and arches led to the former 

 erroneous view that the artery is derived as a small branch from the arch. 



