INTRODUCTION The initial stage in the development of the vertebrate nervous system is the formation of the neural plate from a section of dorsal ectoderm during neurulation. Lateral edges

The initial stage in the development of the vertebrate nervous system is the formation of the neural plate from a section of dorsal ectoderm during neurulation. Lateral edges of the neural plate subsequently protrude to form the neural folds, which eventually fuse at the dorsal midline producing the neural tube. During this stage of morphogenesis, some neural fold cells migrate out of the neural tube dorsally and produce the neural crest beneath the future epidermis. The neural tube lineage gives rise to the central nervous system (CNS), whereas the neural crest lineage produces the peripheral nervous system (PNS) and other derivatives including melanophores. Experimental manipulations such as transplantation or ablation of embryonic portions in amphibians have revealed that the instruction of dorsal mesoderm to overlying ectoderm during gastrulation is required for the formation of the neural plate and neural fold (Spemann, 1938). The part of ectoderm that escapes the influence of the dorsal 1067 Development 119, 1067-1078 (1993) Printed in Great Britain © The Company of Biologists Limited 1993