Definition: Nieuwkoop and Faber stage 15; age 17 hr 30 min at 22-24 Celsius; corresponding to the following external morphological criteria: Early neural fold stage. Presumptive cement gland faintly circumscribed. Anterior part of neural plate roundish. Neural folds distinct, except medio-rostrally; initial formation of sharp inner ridges on neural folds in rhombencephalic region. Length: 1.5 - 1.6 mm.