The neural tube is a structure that develops in all vertebrate embryos and eventually becomes the brain and spinal cord.  This structure must receive certain cues from surrounding cells and tissues in order to form properly.  The most critical step is a “zipping” action that occurs as the neural tube forms and closes, as depicted in the image below.

Image courtesy of https://www.google.com/search?q=neural+tube+formation&rlz=1C5CHFA_enUS758US758&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjNvLP98LPcAhUoh-AKHYS5DrMQ_AUICigB&biw=1405&bih=735#imgrc=TokGPB_rc_clqM: