It is within this stage, when ~28 days have passed since fertilization. The neural plate is flanked by 8 somites. It is transforming into a neural tube at the level of somite 4, representing the future junction of the head and neck. The endoderm, shown in grey, is still continuous at its periphery with the yolk sac, which is shown in darker grey.

Within at most 2 days, the heart has transformed into a single endocardial conduit extending between still paired venous and arterial vessels. The single channel has the embryonic left ventricle as its caudal, and the embryonic right ventricle as its cranial component. The umbilical vein, which occupies the boundary of embryonic disk and amnion, and the vitelline plexus, which is situated on the yolk sac, merge at the level of the 4th somite to form the hepatocardiac channels. These channels, in turn, join the systemic venous inflows to the heart. At this stage, both arms of the cardiac inflow tract are transversely oriented vessels, merging in the midline. This site of union represents the caudal continuity between the first and second heart fields, and corresponds with the future atrioventricular junction. It is not yet possible anatomically to identify specific venous tributaries, but the primordia of the atrial chambers are visible. Cardiac jelly forms a thick cuff around the single endocardial tube, while the outer myocardial wall surrounds the jelly as a cloak, which is open dorsally as the dorsal mesocardium. The dorsal mesocardium connects the heart with the overlying pharyngeal floor, while the transverse septum supports the embryonic ventricle caudally.

The lumen of the heart resembles that of an hourglass. At the narrowest part of the hourglass, the dorsal mesocardium has disappeared. At this site, the transverse pericardial sinus, identifiable by the interruption of the dorsal mesocardium, marks the transition from the descending, or inlet, to the ascending or outlet limb of the forming cardiac loop. This junction between the embryonic left and right ventricles will eventually be the location of the interventricular foramen. It is at this position, furthermore, that the heart tube bends rightward and, in particular in its cranial part, ventrally.

The looping of the heart brings out the separation of the second heart field into caudal and cranial portions. In mice, the caudal second heart field gives rise to both atrial chambers at CS10-11, and the systemic venous sinus at CS12 (Table 1). At CS10, the cranial second heart field gives rise to the embryonic right ventricle proximally, while the distal portion becomes the myocardial outflow tract at CS11 (Table 1). In mice, the embryonic right ventricle originates from myogenic cells in the second heart field, which also give rise to the muscles of the 1st pharyngeal arch, whereas the outflow tract is covered by cardiomyocytes, which originate in similar fashion from myogenic cells in the 2nd pharyngeal arch.

The outflow tract, at the arterial pole, continues extrapericardially as the paired ventral aortas, which extend parallel to the pharyngeal floor in cranial direction. They then pass perpendicularly to the pharynx, in front of its widening part, which will give rise eventually to the pharyngeal pouches, to join both dorsal aortas. Ventral aortas are found in embryos of all higher vertebrates, including human embryos during CS10 and CS11. The cranial boundary of the cardiac jelly coincides with the transition of the outflow tract to the ventral aortas. The dorsal aortas course caudally between the dorsolateral wall of the pharynx and the somites, breaking into a plexus where somites are forming. By this stage, the roots of the first pair of intersegmental arteries can be recognized between somites #1 and #2.