There is still only one pair of aortic arches. The common cardinal vein on the yolk sac is very broad and prominent. Segmental arteries and veins are present along the trunk and tail. These vessels are deep to the myotomes, and are most easily recognized overlying the spinal cord near each transverse myoseptum. Those in the anterior tail begin to carry circulating blood.
Dechorionated embryos lie on their sides when at rest, yolk sac obliquely upwards.
A touch elicits a vigorous and rapid response, with a wriggling rhythm just slow enough that its beat is still barely discernable. The response propels the embryo from a few body lengths in many cases, or up to the whole width of a 5 cm dish in others. At the end of the bout of swimming the embryo comes to rest on its side, not dorsal-up.