ORIGIN OF THE PULMONARY VESSELS IN THE CHICK. 19 



That this outgrowth of endothelium or angioblasts is the first indication of the 

 common pulmonary vein is supported by the following facts: (1) There is no venous 

 opening into the sinus at this point, either before or during the proliferation of 

 angioblasts from the dorsal wall of the sinus venosus. (2) This mass of cells occu- 

 pies the exact position at which, in a later stage, the common vein opens into the 

 sinus venosus. (3) Liquefaction can be seen in this mass of angioblasts before the 

 vein has opened into the sinus. (4) The orifice of the common pulmonary vein 

 in later stages can be seen at this point, the mass of cells having disappeared. 

 (5) The lip of the left valve of the sinus venosus is also derived from these cells and 

 is present throughout the process, having distinct histological differences that 

 render its identification a simple matter (figs. 4, 5). 



Some confusion may arise from the fact that the pulmonary vein opens into 

 the sinus at the left of the left valve of the sinus ; in other words, the left valve lies 

 to the right of the opening of the vein. A study of the early development of the 

 heart shows this to be the case. Later, however, when the left valve fuses with the 

 dorsocaudal extremity of the septum superius (Brown), the opening of the vein is 

 assigned to its final position in the left auricle. 



The pulmonary circulation goes through two phases of development, ascending 

 and retrograde. The former reaches its maximum at the stage of 90 hours' incu- 

 bation. At this time the system consists of two pulmonary arches, two pulmonary 

 arteries, and a common pulmonary vein with four main branches plus connections 

 to both anterior cardinal veins. From this time on, the system may be said to 

 undergo retrograde changes leading to the adult structure. It is beyond the scope 

 of this paper to consider more than the origin of these vessels and the first step in 

 their retrogression, i. e., the loss of two of the branches of the common vein. 



With the formation of the common pulmonary vein and its connection with 

 a patent splanchnic plexus of capillaries over the ventral surface of the gut, a new 

 path of blood-flow is established between the arterial and venous portions of the 

 heart through this plexus. The axis of the common vein is perpendicular to that 

 of the plexus and divides the plexus into two portions, the cephalic and post-caval, 

 both of which drain into the common vein. A change occurs, due to dynamics 

 of circulation and growth, in which the capillaries in each of the four directions about 

 the common vein are replaced by individual vessels that take over the function of 

 the plexus. On the right and left sides of the gut, at the level of the lung-bud, the 

 right and left lateral branches are formed. These are the true pulmonic branches, 

 in that each drains its respective artery in the right and left lung rudiment. They 

 persist and develop with the lungs. 



The capillaries caudal to the common vein begin to disappear early, decreasing 

 in size, number, and importance. They are merely the connections between the 

 cephalic and post-caval portions of the splanchnic plexus. At the stage of 90 

 hours of incubation they are represented by only one or two small twigs which soon 

 disappear. It is of interest to note that the persistence of one of these vessels may 

 give rise to a very unusual anomaly of the pulmonary circulation. Brown gives an 

 excellent description of such a case. 



