In the 
human embryo, 
the
mesodermal germ layer gives rise to the entire cardiovascular system
(heart, blood vessels and blood cells). The heart develops from 
two simple epithelial tubes which fuse to form a single chambered heart
that is efficiently pumping blood by the fourth week of embryonic development.
Twenty-three days following conception, the single, simple epithelial
heart tube lies within the embryo's pericardial cavity. At this time there
are three cell layers present within the heart tube. The inner layer, known
as 
cardiac jelly, is a structureless mass of cells which contain very
few nuclei. The second and third layers are known as the cardiac mantle
and will eventually give rise to the epicardium and myocardium.
The heart tube contains 
three specific areas: the cranial portion, the caudal portion and the
bulbus cordis. As development progresses, the 
cranial one-third of the tube dilates to form the aortic sac which
will give rise to the aortic arches.
The caudal one-third to one half also dilates to form the early 
embryonic ventricle.
The remaining mid-portion forms the bulbus cordis which has three distinct
areas of development. The proximal one-third gives rise to the body of
the right ventricle, which is initially referred to as the primitive ventricle.
The 
remaining two thirds of the bulbus cordis is divided into two sections.
The distal-most section is called the truncus arteriosus which develops
into the aortic root and the parts of the ascending aorta. The remaining
mid-portion is called the 
conus cordis and connects the primitive right ventricle to the truncus
arteriosus.
The conus cordis partitions to form the outflow tracts of the right and left ventricles.
An increase in blood volume occurs following the 23rd day of development.
With this increase in blood, blood circulation changes to a parallel flow.
As the heart tube grows and becomes longer it usually bends to the right,
which is known as
d-looping. Rightward bending is responsible for the initial positioning
of the primitive ventricle.
The cardiac jelly now acts as a valve for movement of blood from the atrial end of the heart tube to the distal end.
By the 24th day of gestation, the primitive ventricles have expanded
and the cardiac jelly contains trabecula or supporting structures. Distal
to the primitive right ventricle is an area known as the 
conotruncal region containing the conus cordis and truncus arteriosus.
The conotruncal region and the primitive right ventricle are collectively
known as the bulbus cordis.
As growth continues, the conotruncal region moves centrally with 
torsion and 
twisting, giving rise to the 
anatomical curve of the 
aorta and the pulmonary artery.