Formation of the General Body structure
Arms and legs first appear at about 28 days after fertilization as limb buds (figure 20.8) and quickly begin to elongate. At about 35days, expansions called hand and foot plates form at the ends of the limb buds. Zones of cell death between the future fingers and toes of the hand and foot plates help sculpt the fingers and toes.
The face develops by fusion of five growing masses of tis-sue, called processes (figure 20.9). One, the frontonasal pro-cess, forms the forehead, nose, and center of the upper jaw andlip. Two maxillary processes form the maxillae (upper lip and jaw), and two mandibular processes form the mandible (lower lip and jaw).
The nose begins as two structures, one on each side of the forehead mass. As the brain enlarges and the face matures, the two parts of the nose approach each other in the midline and fuse (figure 20.9). The two masses forming the upper jaw expand toward the midline and fuse with part of the nose to form the upper jaw and lip. A cleft lip results from failure of these struc-tures to fuse. Cleft lips usually do not occur in the midline, but
The roof of the mouth, or palate, begins to form as vertical shelves of tissue that grow on the inside of the maxillary masses. These shelves swing to a horizontal position and begin to fuse with each other at about 56 days of development. If the palate does not fuse, a midline cleft in the roof of the mouth called a cleft palate results. A cleft palate can range in severity from aslight cleft of the uvula to a fissure extending the entire length of the palate. A cleft lip and cleft palate can occur together, forming a continuous fissure.
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