VALVES IN THE HEAKT OE THE CHICK. 
405 
Fig. 6 Cross section of same truncus arteriosus made in line 2, 2, fig. 4, through lower 
forked margin of the septum between the rudimentary aorta and pulmonary 
artery. 
a. Aortic channel, b. Pulmonary channel. 
Fig. 7. Section of the same truncus arteriosus near its central portion. The slight 
convexity of the posterior surface indicates the position of the prominent 
pyramidal ridge afterwards formed here. 
Fig. 8. Portion of truncus arteriosus at 106th hour of incubation. The section has 
been made between the line 2, 2 in fig. 4, and one a little above the centre 
of the vessel. Figs. 9 and 10 are from the same preparation viewed in dif- 
ferent positions. 
v. Ventricular end of truncus arteriosus, e. Aorta, f. Pulmonary artery. 
g. Lower ends of forked margin of septum. 
Fig. 9. The lower end of the preparation is looked into obliquely so as to show the left 
side and corresponding part of the anterior and posterior surfaces of the inside 
of the vessel. At the upper end the cut end of the left leg of the forked 
septum is seen terminating on the wall of the vessel, and just below and in a 
line with it the rudiment of the anterior aortic valve is seen as a flattened ridge, 
separated from the anterior pulmonary valve-rudiment by a groove expanding 
out below. The under surface of the rudiment of the anterior pulmonary 
valve is seen to the left of this in the preparation. The left leg of the forked 
septum afterwards grows down along the left edge of the rudiment of the 
anterior aortic valve and the adjacent part of the groove between the anterior 
valve-rudiments. 
a. Anterior aortic valve-rudiment, b. Anterior pulmonary valve-rudiment. 
c. Edge of anterior aortic valve along which the septum grows, d. Left lower 
end of forked arterial septum, e. Cut edge of cardiac end of section. 
Fig. 10. Same preparation more highly magnified and turned into a different position. 
The lower end is looked into obliquely, so that the right side and posterior 
part of the inside of the vessel are seen. At the upper aperture of the vessel 
the termination of the right leg of the forked septum is seen. Below this is 
the commencement of the ridge on the posterior surface. It is at this time 
merely a thickened ridge passing obliquely across and down the vessel, from 
the right side towards the posterior surface. Its ends are the rudiments of 
the inner semilunar valves of each artery, while its central portion afterwards 
becomes prominent and pyramidal. The lower face of the ridge slopes off 
rapidly into the wall of the vessel. Between the right end and the rudiment 
of the anterior aortic valve (its corner just seen) a light space is seen in the 
drawing, where the outer aortic valve afterwards becomes developed. 
a. Part of anterior aortic valve, b. Rudimentary ridge on posterior surface. 
c. Space where lateral valve subsequently developes. d. Right lower end of 
