The Folding of the Embryo

Script: Between 3 and 4 weeks, the body plan emerges as the brain, spinal cord, and heart of the embryo are easily identified alongside the yolk sac.

Rapid growth causes folding of the relatively flat embryo. This process incorporates part of the yolk sac into the lining of the digestive system and forms the chest and abdominal cavities of the developing human.