What happens to the hollow nerve cord?
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&dagger; (page 82 onwards in google books; google book links are apparently transient and they expire; therefore I didn't provide one).



&dagger; Haines, Duane E., and M. D. Ard. Fundamental neuroscience for basic and clinical applications. Philadelphia, PA: Elsevier/Saunders, 2013. ISBN: 9781437702941