Which organ develops first in a human embryo?
The process of cell division and cellular differentiation of embryo during the early stages of development is called embryogenesis.
The development of organs start during the third week of embryogenesis and is called organogenesis.
The heart is the first functional organ to develop and starts to beat and pump blood at around 21 - 22 days. It develops as a horse shoe shaped area , near the head of the embryo and consists of cardiac myoblasts and blood islands.