Embryo. A five week old human embryo. Its crown- rump length from the vertex of the skull to the mid-point of the buttocks is about 10-12mm. At this stage the back of the embryo (bottom) grows more rapidly than the front giving the embryo a c- shape. The buds of the limbs are visible as round areas at top left and bottom centre. A rudimentary eye is seen in the head (upper centre right) as a round feature. At five weeks all the internal organs such as the liver, pancreas, heart, and lungs have begun to form; this is the period in which the embryo is at the greatest risk of birth defects due to genetic or external factors.

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