The Stages of Human Embryonic Development

Stage 9 is characterized by the first appearance of somites and prominent neural folds. The rapid growth of the rostral neural folds dominates this stage. It leads to formation of a distinct 'head', the initial appearance of the foregut and the rotation and translocation of the primordial heart from a rostro-dorsal location to a more ventral-caudal location.

Stage 9 embryos have a greatest length of 1.5 to 2.5 mm and an estimated postfertilization age of 24 - 26 days.