Heartbeat -- Stage 23 The heart first begins beating at about 46 hours post fertilization (stage 23). The blood initially has no red blood cells and is colorless and circulation is mainly used for nutrient transfer from the yolk to the embryo proper. In this video segment, the head of the embryo is to the right and a single large oil droplet is immediately below the beating heart. Note that the optic cup has not yet developed a pigmented retina. Also, the vessels in the yolk sac are relatively straight and unbranched. They later branch and become more sinuous.