Heartbeat -- Stage 28 This movie shows a stage 28 embryo (74+ hour post fertilization) from a head-on perspective. At the beginning of the movie, the embryo flexes its tail. Note the beating heart, the blood circulating through the vitelline veins within the yolk sac membrane, and the single large oil droplet. At this stage, the blood now includes red blood cells and has become pink. Full pigmentation of the retina of the eye is now also evident.