A baby's skeleton begins
as fragile membranes and cartilage. As the fetus
develops, the membranes and cartilage turn into
bone in a process called ossification.
During the third month of development, the membranes
on the side and back of the fetus' skull
start to ossify. Bone tissue slowly grows over
the area where the membranes once existed. Eventually,
these bone plates will grow together forming
the cranial cavity which protects the brain.
Close to birth, the bones of the skull still
have gaps between them called fontanelles. The
fontanelles allow room for the baby's brain
to grow and enable the head to be compressed
during delivery.
Most of the bones of the skeleton start off
as cartilage, such as the arms, legs, ribs, fingers,
and backbone. From the second month until the
end of the third month, the cartilage in the
middle of the bones begins to ossify outward.
Bones continue to grow in this manner until adulthood,
allowing them to increase in their length and
width.
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