As to the origin of the pulmonary vein, there are two theories held by the earlier investigators. One holds that the anlage of the pulmonary vein grows out through the dorsal mesocardium toward the lung buds as an endothelial proliferation from the sinus venosus; the other view asserts that this vein arises from an indifferent preexisting plexus which connects the lung buds and the sinus venosus. On the further developmental stages of the vein there are also some interesting problems which were less studied, namely: 1) The connection between the pulmonary vein and the systemic circulation. 2) The position of the opening of pulmonary vein in the heart primordia. 3) The length and the direction of the common pulmonary vein. In an effort to solve these questions, the author has selected the embryos of Uroloncha domestica Flower for investigation. A large majority of the embryos were fixed in Zenker's fluid, and stained with borax carmine. After the subsequent treatment in the usual manner, these specimens were embedded in paraffin, trimmed and cut in transverse serial sections with a thickness of 10 microns. A number of models were prepared according to Born-Peter's method of wax-plate reconstruction. From the results of these experiments, the following summary and conclusions may be made: 1) The anlage of the pulmonary vein first grows out in the embryo 4.0mm. in length with 20 somites as an endothelial proliferation from the sinus venosus. 2) The pulmonary opening is found in the median plane of the sinus venosus at an early stage of development, and when the interatrial septum is formed, the opening migrates from the median plane to the left. Consequently, the pulmonary vein empties into the left atrium dorsally. 3) The pulmonary vein is connected with the systemic circulation by a small branch to the post-caval plexus. 4) The direction of the common pulmonary vein is toward dextro-dorsad from the pulmonary opening, and in the growth of the heart, this direction migrates from dextro-dosad to dorsad. 5) The length of the common pulmonary vein is increased as the pulmonary anlage develops, but at the later stage of development it is decreased and the vein is gradually absorbed into the atrial wall.