SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
An Alizarin stained human fetus of about 14 weeks old showing the early development of bone (ossification). Bone formation begins at the end of the second month of pregnancy, continues through birth & up to adolescence. The fetal skeleton, with the exception of the skull & clavicle (which begin as bone), consists of cartilage. Beginning at eight weeks, sites of ossification appear (near the middle of the future bone) & spread outward replacing the cartilage. At birth the small bones of the hands & feet together with sites known as epiphyses (mainly ends of the limb bones), have still to be replaced by bone.
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