302 THE DEVELOPMENT OF THE CHICK 
the pulmonalis, which les dorsal to the aorta distally, is gradually 
The third division of the aortie- 
pulmonary septum arises near the opening of the bulbus into 
transposed to its left side. 
the ventricle in the form of two ridges of the endocardium on 
the right and left sides respectively of the bulbus, the pulmonary 
division lying ventral and the 
division the 
partition. A third 
slight endocardial ridge of the 
proximal part of the bulbus is 
described (Hochstetter, Greil) 
at this stage, but it soon dis- 
The proximal bulbus 
aortic dorsal to 
incipient 
Fia. 201. — A. Section through the appears. 
truncus arteriosus of an embryo of 5 
mm. head-length. 
B. Section through the distal por- 
ridges may be seen on the fifth 
day; on the sixth day they are 
well formed; on the seventh day 
tion of the bulbus arteriosus of the 
same embryo. (After Greil.) 
A., Aorta. P., Pulmonalis. A.S. ao. 
p., Plane of the septum aortico-pulmo- 
nale. 1, 2, and 3, Ridges prolonging 
the septum aortico-pulmonale. 
they have united to form a par- 
tition which becomes continu- 
ous with the partition in the 
distal 
Thus the separation of the aor- 
portion of the bulbus. 
tic and pulmonary trunk is completed down to the ventricle. 
The semilunar valves arise by excavation of three endocar- 
dial thickenings in each trunk formed at the caudal end of the 
distal division of the bulbus (Hochstetter, Greil). The origin 
of these thickenings is as follows. Both the aortic and pulmonary 
trunks receive one each of the original endocardial ridges of the 
distal portion of the bulbus owing to the course of the aortic- 
Mach also receives half of the ridge along 
A third ridge 
A cavity then 
pulmonary septum. 
which the septum of the truncus is prolonged. 
arises subsequently in each between these two. 
arises in each ridge and opens distally into the aorta and pul- 
monary artery respectively, thus forming pockets open in front. 
These valves are fully formed at eight days. 
The aortic-pulmonary septum becomes thick early in its 
history and the muscular layers of the vascular trunks, which 
at first form a common sheath for both, gradually constrict into 
the septum, and separate when the constriction brings them 
together, so that each vessel obtains an independent muscular 
wall. Subsequently, a constriction extends from the outer layer 
