the nasal pits from the oral cavity. During the seventh week the oronasal membrane ruptures and disintegrates to form an opening - the single primitive choana. The intermaxillary segment extends posteriorly to form the primary palate which makes up the floor of the nasal cavity. During the eighth and ninth weeks a pair of thin extensions form from the medial walls of the maxillary process. These extensions are called the palatine shelves that form the secondary palate. The secondary palate will endochondrally ossify to form the hard palate - the end-stage floor of the nasal cavity. During this time ectoderm and mesoderm of the frontonasal process produce the midline septum. The septum grows down from the roof of the nasal cavity and fuses with the developing palates along the midline. The septum divides the nasal cavity into two nasal passages opening into the pharynx through the definitive choanae. At ten weeks, the cells differentiate into muscle, cartilage, and bone. Problems at this stage of development can cause birth defects such as choanal atresia (absent or closed passage), facial clefts and nasal dysplasia (faulty or incomplete development) or extremely rarely polyrrhinia the formation of a duplicate nose. Normal development is critical because the newborn infant breathes through the nose for the first six weeks, and any nasal blockage will need emergency treatment to clear. Development of the paranasal sinuses The four pairs of paranasal sinuses - the maxillary, ethmoid, sphenoid, and frontal, develop from the nasal cavity as invaginations extending into their named bones. Two pairs of sinuses form during prenatal development and two pairs form after birth. The maxillary sinuses are the first to appear during the fetal third month. They slowly expand within the maxillary bones and continue to expand throughout childhood. The maxillary sinuses form as invaginations from the nasal sac. The ethmoid sinuses appear in the fetal fifth month as invaginations of the middle meatus. The ethmoid sinuses do not grow into the ethmoid bone and do not completely develop until puberty. The sphenoid sinuses are extensions of the ethmoid sinuses into the sphenoid bones. They begin to develop around two years of age, and continue to enlarge during childhood. The frontal sinuses only develop in the fifth or sixth year of childhood, and continue expanding throughout adolescence. Each frontal sinus is made up of two independent spaces that develop from two different sources; one from the