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The os incisivum is the homologue to the premaxilla and is incorporated into the development of the maxilla in the fetal skull. The simplest scenario involves a single ossification center arising in the body of the maxilla and spreading to form the entire bone. These centers appear during the sixth embryonic week and join by the third month. By late fetal life there exists no clear indication of separate centers on the facial side of the bone however, on the palate approximately 25 percent of skulls retain a suture line at the junction of the two centers. This suture is sometimes retained in adult skulls.