Human embryo. Illustration of a human embryo after approximately 33 days. After around 33 days the embryo is not yet recognisably human and measures only around 5mm in length. The developing ears appear as pits (upper centre) and rudimentary eyes develop in the form of stalks. The nose and mouth will develop from the folds in the neck. Below the neck is the heart bulge; at this stage the immature heart already produces a form of heartbeat. Rounded buds form the beginnings of arms and legs and internal organs are growing. The back grows more rapidly than the front in the young embryo, giving it a C-shape and a conspicuous tail.