Human embryo at 6 weeks gestation, measuring approximately 13 mm crown to rump. The upper limbs, fingers, and face are developing and growing rapidly but are still forming their shape. The digital rays which will become fingers can be seen in the developing hand. The eye is developing but is not yet functioning, and the brain is developing rapidly. Note the yolk sac, the umbilical stalk, and the developing notochord along the back that will become the spine.