Most long-pec embryos have other iridophores associated with the dorsal stripe in the tail, and occasional feebly reflective ones that appear on the dorsolateral yolk sac. The shrinking yolk sac makes the pericardial cavity conspicuous. The vigorous beat of the heart easily shows that the heart tube is bending to bring the atrium to a position dorsal to the ventricle. Nomarski optics reveal that the definitive six pairs of aortic arches are now all present, but typically at the long-pec stage not all of them are carrying circulating blood. Generally, the more anterior arches, especially aortic arches 1-3, join the circulation earlier, but participation varies.
Circulation is now strong in most segmental vessels along the trunk and tail.
There is a single channel associated with each segment, but, as can be determined by following the course of the blood flow, an artery in one segment leading from the dorsal aorta alternates with veins in the adjacent segments leading to the axial vein. There is no blood circulation in the pectoral fin bud until about 52 h, more consistently at 54 h. At this point one can find the subclavian vessel looping around the bud at the base of the ridge, and without a clear distinction between the arterial and venous side of the loop. The common cardinal vein on the yolk ball is fan-shaped, narrower at its origin and broadening more ventrally. When at rest, dechorionated embryos are beginning to assume a dorsal-up body position.