20 ORIGIN OF THE PULMONARY VESSELS IN THE CHICK. 



A most interesting vessel is derived from the capillaries cephalad to the com- 

 mon vein, i. e., the cranial tributary of the pulmonary vein. This lies in the mid- 

 line of the ventral surface of the gut and drains a system of anastomoses between 

 the two pulmonary arteries, receiving also small twigs from the pulmonary arches. 

 Figure 10 shows this vessel at the height of its development. It, also, is a temporary 

 structure and begins to degenerate at the stage of 100 hours. Squier has shown 

 a later stage in which it has lost its rich arterial connections and stands out like a 

 dead branch of a tree, finally disappearing. Squier used a method of wax recon- 

 struction and described a stage 10 hours older than that shown in figure 10. During 

 this period several changes take place. The cranial tributary loses its connections 

 with the pulmonary arteries and disappears. The distal communications with the 

 post-caval plexus have disappeared. The lung rudiments begin to show definite 

 signs of lobulation and the vascular picture has accommodated itself to that change. 



In summary, then, the formation of the pulmonary circuit falls into three main 

 periods : 



(1) Precirculatory.A proliferation of angioblastic cells from established 

 embryonic endothelium, with the possibility also that some of the vasoformative 

 cells may differentiate from mesoderm and join in the process. This mesh of 

 angioblasts undergoes cytoplasmic liquefaction, forming a capillary net over the 

 surface of the primitive gut. From this plexus the pulmonary vessels are evolved. 



(2) Circulatory. After the capillary plexus is patent, a new route is estab- 

 lished between the arterial and venous portions of the heart. The plexus undergoes 

 a change in pattern with the establishment of new lines of blood-flow and the forma- 

 tion of definite vessels, such as the pulmonary arches, arteries, capillaries, and veins. 



(3) Adaptive. With the development of the lung, new patterns of vessels 

 are evolved to accommodate the circulation to this change. This leads to the 

 formation of a true pulmonary circulation. The arteries increase in length, the 

 capillaries over the lung rudiments increase in number, and the remnants of the 

 indifferent plexiform stage disappear. The cranial tributary has reached its highest 

 development and is about ready to disappear. The post-caval connections have 

 already disappeared except for one or two small remaining twigs. 



Streeter (1915), in a study of the vascular system of the brain of the human 

 embryo, divides the stages of development of the brain-vessels into five periods, 

 showing the various adaptive changes which the circulation goes through in accom- 

 modating itself to the ever-changing environment of embryonic development. 



PULMONARY ARTERY. 



The recent views on the origin of the pulmonary artery have undergone a 

 complete change from the old concepts that still dominate the text-books, based 

 on the works of His, Zimmermann, Rathke, and others. The old idea that the 

 pulmonary artery is derived as a branch from the pulmonary arch was the ac- 

 cepted one until the recent work of Fedorow, Bremer, and Huntington. Even 

 Bremer (1902, 1909) adhered to this conception in his first two articles, but cor- 

 rected it in a third paper on the rabbit embryo. He describes the origin as a blind 



