EDELMANN / SCIENCE PHOTO LIBRARY EDELMANN / SCIENCE PHOTO LIBRARY
A human embryo after 4 weeks development. The embryo is about 4 millimetres in length from head (top) to tail. At this stage the tubes that comprised the early heart have fused into a common tube in which contractile activity (an early form of heartbeat) is starting. Primordial organs of the thyroid, lungs, liver, pancreas and kidney have been established and are developing.
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