ABSTRACT

The Phaiynx The embryo s pharynx (throat) at first lies cranio-dorsal to the heart (4 weeks), extending from the oropharyn­ geal membrane in the depths of the oral pit, to the midgut (Figure 1). Lateral to the pharynx, the body wall is corrugated by pharyngeal clefts 1, 2, 3, 4, demarcating successive pharyngeal arches I, II, III, IV and VI (Figure 2). The ectodermal pharyngeal clefts are fused with endodermal pharyngeal pouches, forming imperforate pharyngeal septa between the mesodermal arches (Figure 2). Bilateral ectodermal thickenings or placodes lie on the side of the head; the nasal placode in the roof of the oral pit, the lens placode lateral to the diencephal­ ic part of the neural tube, and the otic placode dorsal to clefts 2 and 3 (Figure 1).