To demonstrate this, I present three illustrations below. Figures 1 and 2 demonstrate the formation of the two lateral endothelial tubes in an early embryo. The horse shoe shaped tubes in figure 2 forms from coalescing angiogenic cells shown in figure 1. The two lateral tubes approximate each other as the embryo folds laterally, and eventually fuse to form a a single, ventrally located heart tube. The images are as clear as they can possibly be, however visualizing such events is extremely difficult due to a three dimensional nature which changes by time.
Angiogenic cells form in a horse shoe distribution lateral and rostral to the brain plate.
Angiogenic cells coalesce to form two lateral and two median endothelial tubes. The two lateral tubes come close together as the embryo folds to fuse and form the single heart tube. The animation clearly illustrates the process described above. Instructions to view the animation Click on the image on the right to view. This is a Quick Time animation and will take a short while to download.