Cephalic Neural Crest and Placodes
In 3-4 week human embryos, the ectoderm covering the head shows considerable re gional differences in both structure and thickness. The temporary appearance of circumscribed areas of relatively thick ectoderm has been observed in embryos of a variety of vertebrates. These areas, discovered by Van Wijhe (1882) in fish embryos, were called placodes by Von Kupffer (1894). These placodes have ...