Neurulation
Neurulation, the formation of the central nervous system, is different in fishes than in most other chordates. The neural tube begins as a solid cord formed from the ectoderm. This cord then sinks into the embryo and becomes hollow, forming the neural tube. This process contrasts with the process in other chordates, which occurs by an infolding of the ectoderm to form a hollow tube.
Read more about this topic: Fish Development