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Foetal skeleton. Twelve-week-old human foetus stained to show bone development. The crown-rump length (sitting height) of the foetus was 26mm. The head (at top) constitutes about one half of the length at this stage. Ossification (bone formation) is in progress and the partially ossified bones of the skull, arm and ribcage are seen (red). The face has all the human characteristics but the ears are still primitive. Movements begin at this time but are usually not felt by the mother. At twelve weeks most of the internal organs have formed and the sex may be determined by external (ultrasound) examination. Alizarin stained.
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