VALVES IN THE HEAKT OE THE CHICK. 
411 
septum between the aorta and pulmonary artery becomes united with the 
lower half of the right ventricular border of the aperture in the septum of 
the ventricles, and thus completes the separation of the aortic from the 
pulmonary infundibulum. The semilunar valves are omitted for the sake 
of clearness. The dotted line shows the part which afterwards becomes 
closed up. 
a. Cut surface of truncus arteriosus, b. Cut edge of ventricle, c. Inter- 
ventricular septum, d. Left ventricle, e. Eight ventricle, f. Aorta, g. Pul- 
monary artery, h. Orifice in septum becoming the opening of the aorta into 
the left ventricle, i. Opening of right ventricle into pulmonary artery. 
j. Dotted line showing position of hourglass-shaped aperture. Jc. Forked 
lower border of arterial septum. 
Fig. 39. Imaginary sections of the truncus arteriosus made just above the semilunar 
valves at different periods of development, showing the gradual changes in 
the position of the roots of the aorta and pulmonary artery during the process 
of the division of the truncus arteriosus, and their position at its termination. 
1 . At commencement of process of division of the truncus arteriosus. 5 . At 
termination of same. 2, 3, 4. At intermediate stages, a. Aorta, b. Pul- 
monary artery. 
3 I 2 
