The ribs begin to develop in the mesoderm during the sixth week of prenatal life:
Anomalies can occur in the number and form of ribs. Otherwise physically healthy people can possess between 11 and 14 pairs of ribs. Infrequently, both lumbar and cervical accessory ribs occur due to the overdevelopment of the costal process of the transverse processes of L1 and C7 respectively.
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