410 
DR. M. TONGE ON THE DEVELOPMENT OE THE SEMILUNAR 
artery have been cut across just above the semilunar valves, and the ventricles 
cut off just below them. 
A. Aorta. P. Pulmonary artery, a, a. Lateral valves, b , b. Inner valves. 
c, c. Anterior valves, d, d. Cut edge of arterial wall, e, e. Muscular sub- 
stance of ventricle. 
Fig. 33. Heart and great vessels of Chick at the end of the 13th day of incubation. 
The muscular substance of the front of the ventricles has been removed. 
The septum and apex are left. 
a. Aorta, p. Pulmonary artery. 1. v. Left ventricle, r. v: Right ventricle. 
m. v. Mitral valve. 
Fig. 34. shows the distal portion of the truncus arteriosus and its principal branches at 
the 140th hour of incubation. The truncus arteriosus has been cut across 
just above the rudimentary semilunar valves. The shaded portions are the 
clots in the vessels. The right side of the preparation is here seen. 
a. Truncus arteriosus, b. Aorta, c. Pulmonary artery, d. Origin of 
branch to right lung. e. Descending aorta, f. Ductus arteriosus, g. Por- 
tion of third branchial artery between carotid and fourth branchial artery 
(descending aorta) nearly obliterated. 3, 4, 5. Third, fourth, and fifth 
branchial arteries (right side). 
Fig. 35. Front view of same preparation. Part of its left side is also seen. Owing to 
the absence of colour in the clots in the fourth vascular arch and left branch 
of the pulmonary artery, the latter vessel is not traceable to its termination, 
and the fourth arch seems to be obliterated. 
i. Left carotid. 1. Arch of aorta. Other letters the same as in fig. 34. 
Fig. 36. Left side of same preparation. Outline of left branch of pulmonary artery 
only just traceable to its termination in descending aorta. Fourth vascular 
arch apparently obliterated. 
Letters the same as in figs. 34 & 35. 
Fig. 37. Diagram of magnified truncus arteriosus about 106th day of incubation. 
The forked septum between the fourth and fifth pairs of branchial arteries is 
seen just extending into the truncus arteriosus. The dotted lines indicate 
the directions taken by the right and left legs of the fork in their growth down 
the vessel. 
a. Rudimentary anterior valves, b. Commencement of ridge on posterior 
surface of vessel, c. Forked septum growing into vessel between fourth and 
fifth pair of branchial arteries, d. Thickening of truncus arteriosus at ven- 
tricular opening. 3, 4, 5. Third, fourth, and fifth pair of branchial arteries 
respectively. 
Fig. 38. Diagram of upper portion of septum of ventricles and base of the trun- 
cus arteriosus, to illustrate the way in which the lower border of the forked 
