largely formed by the fusion of these five facial prominences. The frontonasal process gives rise to the bridge of the nose. The medial nasal processes provide the crest and the tip of the nose, and the lateral nasal processes form the alae or sides of the nose. The frontonasal process is a proliferation of mesenchyme in front of the brain vesicles, and makes up the upper border of the stomadeum. During the fifth week the maxillary processes increase in size and at the same time the ectoderm of the frontonasal process becomes thickened at its sides and also increases in size, forming the nasal placodes. The nasal placodes are also known as the olfactory placodes. This development is induced by the ventral part of the forebrain. In the sixth week the ectoderm in each nasal placode invaginates to form an indented oval-shaped pit, which forms a surrounding raised ridge of tissue. Each nasal pit forms a division between the ridges, into a lateral nasal process on the outer edge, and a medial nasal process on the inner edge. In the sixth week the nasal pits deepen as they penetrate into the underlying mesenchyme. At this time, the medial nasal processes migrate towards each other and fuse forming the primordium of the bridge of the nose and the septum. The migration is helped by the increased growth of the maxillary prominences medially, which compresses the medial nasal processes towards the midline. Their merging takes place at the surface, and also at a deeper level. The merge forms the intermaxillary segment, and this is continuous with the rostral part of the nasal septum. The tips of the maxillary processes also grow and fuse with the intermaxillary process. The intermaxillary process gives rise to the philtrum of the upper lip. At the end of the sixth week the nasal pits have deepened further and they fuse to make a large ectodermal nasal sac. This sac will be above and to the back of the intermaxillary process. Leading into the seventh week, the nasal sac floor and posterior wall grow to form a thickened plate-like ectodermal structure called the nasal fin. The nasal fin separates the sac from the oral cavity. Within the fin, vacuoles develop that fuse with the nasal sac. This enlarges the nasal sac and at the same time thins the fin to a membrane - the oronasal membrane that separates