The initial appearance of ossification in staged human embryos.

Abstract

AbstractSerial sections of 36 embryos aged six to eight postovulatory weeks (12 to 31 mm C. R. length) were examined. The clavicle and the mandible began to ossify during stage 18, 19, or 20, the clavicle (at least frequently) by means of two centers. The maxilla followed rapidly (stages 19, 20). The premaxilla was in continuity with the maxilla proper, either as a mesenchymal condensation or as an ossific area (stages 20, 21). The next elements to commence ossification were the humerus (stages 21, 22), the radius (stages 21, 23), and the femur, tibia, and ulna (stages 22, 23). By stage 23 the following were also undergoing ossification in at least some cases: interparietal, supra‐occipital, anterior process of the malleus, the squamous temporal, palatine, medial pterygoid plate, zygomatic, frontal, vomer, scapula, and distal phalanges of the hand. Cellular invasion was found in the clavicle and in the bone collars of the radius, ulna, and femur at eight postovulatory weeks. The cartilages of the first pharyngeal arches (chondrifying at stages 18, 19) approached the median plane (stage 20) but did not fuse in the embryonic period proper; their cartilaginous tissue underwent localized hypertrophy by stage 23, but endochondral ossification did not occur until the fetal period.