During this period, the body continues to lengthen and the pharyngeal arches (after which the stage is named) become evident. The arches are a series of structures located in the head ventral to the midhindbrain region, beneath the otic vesicle (the structure that makes the inner ear). The first arch forms the mandible, the second the hyoid cartilage, and the subsequent arches the gills. An accurate way to stage the embryo during this period is to use Nomarski optics to locate the tip of the caudally migrating lateral line primordium (37). This has reached a precise somite location by a specific time. In addition, the fins begin to grow, and the brain undergoes a compression such that the otic vesicle lies closer to the eye. The heart begins to beat at the beginning of the period, and the vascular system develops. Finally, pigment cells begin to differentiate and are evident over the body and in particular sites, such as the pigmented retinal epithelium (Fig. 2I).
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