STAGE 13 ½. Age 15 ½ h.; length 1.5 -1.6 mm.

Ext. Crit.: *Initial neural plate stage*. Neural plate clearly delimited*.

Int. Crit.: First indication of formation of neural crest zone in margin of anterior half of neural anlage. Continuous myocoelic slit in presumptive somite mesoderm. *Archenteron widened and extended to definitive position at about 45° ventral to animal pole*; first formation of liver diverticulum.