Around day 16 or 17, the mesoderm thickens near the cephalic end, forming a central channel that ultimately becomes the heart and great vessels. The heart begins to pump plasma through the vessels on day 20, and on day 21 fetal RBCs appear. These very immature, nucleated RBCs are soon replaced by mature forms. Fetal vessels soon develop throughout the body. The allantoic sac atrophies rapidly, and the body stalk becomes the umbilical cord, connected to vessels in the placenta.
Umbilical vessels carry blood to and from the placenta.
Most malformations occur during the first 12 wk, when external teratogenic influences, such as the rubella virus, are most destructive. All rights reserved.