Heartbeat -- Stage 36 Here the embryo is seen head-on and upside-down. Contraction of the atrium and ventricle are clearly distinct. At this stage, the heart has rotated so that the atrium is now slightly above and to the left of the ventricle. The blood is red. Note the diminished size of the yolk sac and the remaining oil drop. The heartbeat rate is now about 170 beats per minute. Hatching of the embryo is imminent.