What happens to the hollow nerve cord? The dorsal nerve cord of vertebrates is a hollow structure that develops into the nervous system. This embryonic tissue is hollow and I wonder what happens to this 'hollow' later? Does it form the ventricles of brain?

The "hollow" i.e. the neural canal develops into the ventricles, the cerebral aqueduct and the spinal canal (Wikipedia; also see this site).

For a more authoritative reference, see this book by Haines and Ard† (page 82 onwards in google books; google book links are apparently transient and they expire; therefore I didn't provide one).

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† Haines, Duane E., and M. D. Ard. _Fundamental neuroscience for basic and clinical applications._ Philadelphia, PA: Elsevier/Saunders, 2013. ISBN: 9781437702941