A thick, U-shaped primordium positioned at the border of the future epidermis and the neural plate/neural crest forming regions, in the neural stage embryo. This field of naive ectoderm is subdivided into 3 domains, each with distinct gene expression profiles: the anterior PPE (bottom of the U), the lateral PPE (the curves of the U), and the posterior PPE (arms of the U and adjacent to the neural crest/neural plate border). The PPE differentiates into the sensory placodes, which in turn form the adenohypophysis/anterior pituitary gland and sensory organs (such as the olfactory epithelium, lens, trigeminal nerves, epibranchial nerves, otic vesicles and lateral lines).