Very early in the development two paired intraembryonic systems appear:
the superior cardinal veins (vv. cardinales anteriores) bring the blood from the head region via the left and right common cardinal vein.
The inferior cardinal veins (vv. cardinales posteriores) drain the blood from the lower half of the body into the common cardinal vein. From here it empties via the sinus horns into the sinus venosus and into the atrium.
In addition, a further mesh of veins, the subcardinal vein, appears on both sides behind the nephrogenic cord with the formation of the urogenital system.

Soon not only do anastomoses arise in the vertical direction on both sides of the aorta, but also between the left and right subcardinal vein. These anastomoses are found in front of the aorta dorsalis.

The right subcardinal vein obtains access to the vessels of the liver (v. revehentes) that have broadened on the right side to form a hepatocardiac canal.
With the begin of limb development a venous runoff from arm and leg anlagen is formed.

Like the cardiac sinus venosus the entire venous system of the lower half of the body is also located towards the right. A further longitudinal system arises. It is called the supracardinal veins and has a close relationship with the truncus sympathicus.

Now the inferior cardinal veins slowly atrophy on both sides. What remains is a vein plexus that gets larger and larger in the sacral area, the sacrocardinal plexus. The supracardinal vein is also involved. Now the movement of the venous runoff towards the rights is clearly visible.

The inferior cardinal vein and the anastomosis to metanephros still exist only in the caudal part of the embryo. Further along, a connection between the right supracardinal vein and the right subcardinal vein arises.
The connection of the right subcardinal vein with the liver veins has extended.
Near the heart the left sinus horn as well as the left omphalo-mesenteric vein atrophy. Also in the upper area of the body the vein system is moved to the right. A brachiocephalic anastomosis forms between the left and right superior cardinal vein.

The inferior cardinal veins now only exist near the heart and in the sacral region. Between them a prominent intersubcardinal anastomosis net has formed, out of which the kidney veins will be engendered among other things.
Also the supply for the two suprarenal glands as well as the gonad anlagen will arise from it.

Here one can clearly see that the inferior vena cava is put together from several sections of various vein systems. In the abdominal region there are three sets of paired veins: the renal vein, the suprarenal vein and the testicular or ovarian vein. Nevertheless, the blood from the left gonadal vein (testicular/ovarian) does not flow into the v. cava but, due to its location on the right, but into the left renal vein.
In the sacral region a median sacral vein has formed from the sacral plexus. In further development, the pelvic veins also are formed from it.
An anastomosis forms below the liver between the two supracardinal veins out of which a part of the hemiazygos vein will arise.

At the end of the embryonic period the vein system has nearly reached the condition of an adult.
The right supra- and subcardinal vein as well as the liver veins, that form the liver-heart-canal, are involved in forming the inferior vena cava.
In the sacral region the common iliac vein as well as the internal and external iliac vein have formed out of the sacral plexus following diverse transformational processes.
It is to be said again that the gonad vein pair do not have their inlets at the same place.