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TEXT-BOOK OF EMBRYOLOGY. 



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 re- 

 mains free, leaving an opening between the two ventricles (Figs. 202 and 203). 

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



Aorta -I 



Aortic septum 



Interventricular opening 



Right atrio-ventricu- 

 lar orifice " 



Right ventricle " ~ ? 

 Ventricular septum 



Pulmonary artery 



- Aorta 



Left atrio-ventricular orifice 



- Left ventricle 



Fig. 203. — Ventricles and proximal ends of aorta and pulmonary artery of a 7.5 mm. human 

 embryo. Lower walls of ventricles have been removed. Kollmann's Atlas. 



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. 204) — 

 gradually grows toward the heart through the aortic bulb and finally unites with 



Fig. 204. — Diagrams representing the division of the ventral aortic trunk into aorta and 

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



the ventral edge of the ventricular septum, thus closing the opening between the 

 two ventricles. Corresponding with the edges of the septum aorticum, a groove 

 appears on each side of the aortic trunk and gradually grows deeper and ex- 

 tends toward the heart, until finally the trunk and aortic bulb are split longitudi- 



