The face develops between the fourth and eighth week of embryonic development from structures including the frontonasal process, medial nasal process, and maxillary process. The upper lip forms from the maxillary processes and medial nasal processes fusing, while the lower lip forms when the mandibular processes fuse in the midline. Other structures including the nose, cheeks, eyes, and ears develop from the merging and shaping of embryonic processes during this period. Abnormalities can occur if formation of these structures is disrupted.