Protochordates amphioxus

The protochordates (amphioxus and tunicates) occupy a pivotal position in chordate phylogeny, being the closest living invertebrates to the vertebrates. In spite of their evolutionary significance, these animals do not feature commonly in modern developmental biology research. This has not always been the case indeed, amphioxus ranked as one of the principal animals for embryological description in the early part of this century. The ascidia (one group of tunicates) have received intensive study as a model for determinative development, and considerable experimental and molecular data have been accumulated over the past few decades (1). 

There is no unequivocal evidence for single cells migrating from the neural plate or neural tube of protochordates. Some authors have suggested that lateral movement of dorsal epidermal cells towards the midline during amphioxus neurulation may be compared to the migratory behaviour of vertebrate neural crest cells (Holland et al. 1996). However, amphioxus cells move as an epithelial cell sheet and represent a fundamentally different cell behaviour from that shown by mesenchymal neural crest... 

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