LATER DEVELOPMENT OF VASCULAR SYSTEM dol 
appropriate valves are formed. We have thus to describe the 
origin of three primary septa, viz., the interauricular septum, 
the interventricular septum, and the septum of the truncus and 
bulbus arteriosus. These do not, however, themselves unite 
directly, but are joined together by the intermediation of a fourth, 
large, cushion-like septum formed in the auricular canal, 7.e., in 
the opening between the primitive atrium and ventricle. 
In general it may be said that the development of the three 
primary septa takes place from the periphery towards the center, 
7.e., towards the cushion-septum of the auricular canal, and that 
it is practically synchronous in all three, though there is a slight 
precedence of the interauricular septum. During the same time 
the cushion-septum of the auricular canal is formed. We may 
then consider first the origin of these septa separately, and second 
their union. 
(a) The Septum Trunci et Bulbi Arteriosi (Septum Aortico- 
Pulmonale). This septum divides the truncus and bulbus arte- 
riosus into two arteries, the aorta and pulmonary artery. Three 
divisions may be distinguished, viz., a part in the truncus arte- 
riosus, a part in the distal division of the bulbus extending to 
the place of formation of the semilunar valves, and a part in the 
proximal portion of the bulbus, which subsequently becomes 
incorporated in the ventricles. In mode of formation these are 
more or less independent, though they unite to form a continuous 
septum. 
The septum of the truncus arteriosus arises on the fifth day 
as a complete partition extending from the cephalic border of 
the two pulmonary arches into the upper portion of the bulbus 
arteriosus; the blood current flowing through the bulbus that 
passes behind this partition enters the pulmonary arches exclu- 
sively, that passing in front enters the two remaining pairs of aortic 
arches. During the latter half of the fifth day and on the sixth 
day the septum of the truncus is continued into the proximal por- 
tion of the bulbus and divides it in two stems. Here, however, 
it co-operates with three longitudinal ridges of the endocardium 
of the bulbus, one of which is in the direct line of prolonga- 
tion of the septum of the truncus, which therefore is continued 
along this one and between the other two as far as the place of 
formation of the semilunar valves (Fig. 201). The entire septum 
thus formed has a slightly spiral course, of such a nature that 
