408 
DR. M. TONGE ON THE DEVELOPMENT OE THE SEMILUNAR 
riosus. h. Future base of outer pulmonary valve, c. Future base of anterior 
pulmonary valve, d. Future base of anterior aortic valve, e. Future base 
of outer aortic valve. 
Fig. 23 shows the upper surfaces of the valves of the aorta and pulmonary artery at 
the 140th hour of incubation. The outer valve of the pulmonary artery is 
here developing into two valves. 
a. Aorta, b. Pulmonary artery, c, c. Anterior valves, d, d. Lateral valves. 
e, e. Inner valves. 
Fig. 24. Heart and truncus arteriosus of Chick at the 144th hour of incubation. The 
auricles and part of the ventricles removed. Eight side of the preparation. 
A small coloured clot is seen in the aorta. The upper surfaces of the aortic 
valves are seen to be nearly horizontal, and still at a considerable distance 
from the bases of the ventricles. The aortic portion of the channel of the 
undivided part of the truncus arteriosus is seen leading down to the opening 
in the ventricular septum, between the edges of the anterior and inner aortic 
valves. 
a. Aortic valves, b. Channel between edges of aortic valves, c. Clot in 
aorta, d. Cavity of right ventricle, e. Muscular substance of right ven- 
tricle. 
Fig. 25. Portion of truncus arteriosus of Chick at the 144th hour of incubation included 
between sections just above the semilunar valves and about the same distance 
from the base of the ventricles, turned so as to show the upper surfaces of the 
valves. These are seen to be three in each artery, and close the canal of each 
vessel pretty accurately. 
a. Aorta, b. Pulmonary artery. 
Fig. 26. The same preparation turned so as to show its under surface. 
a. Aorta, b. Pulmonary artery, c. Founded ridge on posterior aspect. 
d. Edge along which arterial septum grows anteriorly. 
Fig. 27. Heart and great vessels of Chick at the 147th hour of incubation. The auricles 
have been removed, as well as the posterior part of each ventricle. The 
valves completely close the canal of the vessel, a thread-like channel remaining 
between their edges. 
a. Aorta, b. Pulmonary artery, c. Eight branch of pulmonary artery, 
d. Left branch of pulmonary artery, e. Aortic valves, f. Pulmonary valves. 
g. Septum of ventricles, h. Eight ventricle, i. Left ventricle, j. Aperture 
in septum of ventricle leading from left to right ventricle. 
PLATE XXXII. 
Fig. 28. The same preparation turned so as to show the anterior surface of the heart 
instead of the posterior. The branches of the aorta are seen passing in front 
