THE DEVELOPMENT OF THE VASCULAR SYSTEM 235 



septum appears and gradually grows across the cavity forming the septum 

 ventriculorum (Figs. 209 and 210). This septum is situated nearer the right 

 side and is indicated on the outer surface by a groove which becomes the 

 sulcus longitudinalis anterior and posterior. The dorsal edge of this septum 

 finally fuses with the septum dividing the atrio-ventricular canal, but for a 

 time its ventral edge remains free, leaving an opening between the two 

 ventricles (Figs. 211 and 212). 



This opening then becomes closed in connection with the division of the 

 aortic bulb and ventral aortic trunk. On the inner surface of the aortic 

 trunk, at a point where the branches which form the pulmonary arteries 

 arise, two ridges appear, grow across the lumen and fuse with each other, 

 thus dividing the vessel into two channels. This partition the septum 

 aorticum (Fig. 213) gradually grows toward the heart through the aortic 

 bulb and finally unites with the ventral edge of the ventricular septum, thus 

 closing the opening between the two ventricles. Corresponding with the 



FIG. 213. Diagrams representing the division of the ventral aortic trunk into aorta and 

 pulmonary artery- and the development of the semilunar valves. Hochstetter. 



edges of the septum aorticum, a groove appears on each side of the aortic 

 trunk and gradually grows deeper and extends toward the heart, until finally 

 the trunk and aortic bulb are split longitudinally into two distinct vessels, 

 one of which is connected with the right ventricle and becomes the pulmonary 

 artery, the other with the left ventricle and becomes the proximal part of the 

 aortic arch (Fig. 212). The result of the formation of these various septa is 

 the division of the entire heart into two sides. The atrium and ventricle 

 of each side are in communication through the atrio-ventricular foramen, the 

 two sides are in communication only by the foramen ovale which is but a 

 temporary opening. 



After the opening of the sinus venosus is shifted to the right atrium, the 

 left atrium for a short period has no vessels opening into it. As soon, how- 

 ever, as the pulmonary veins develop, they form a permanent union with the 

 left atrium (Fig. 211). At first two veins arise from each lung, which unite 

 to form a single vessel on each side; the two single vessels then unite to form 

 a common trunk which opens into the left atrium on the cephalic side. As 



