STAGE 11 ½. Age 12 ½ h.; length 1.4 -1.5 mm.

Ext. Crit.: *Large yolk plug stage*. *Blastopore groove closed ventrally*.; yolk plug not yet quit circular. At ventral side concentrated superficial pigment still visible. *Diameter of yolk plug about 1/3 of diameter of egg (+/- 40°).

Int. Crit.: *First indication of neural anlage in sensorial layer of ectoderm*; epithelial layer now a flattened epithelium. *Mesodermal mantle extending dorsally 20-30° from animal pole*, ventrally not beyond equator. *Archenteron extending over 80-90°, still slit-shaped*. Entodermal mass displaced toward ventral side.