STAGE 10 ½. Age 11 h.; length 1.4 - 1.5 mm.

Ext. Crit.: *Cresent-shaped blastopore stage*. Blastopore groove angular. Vegetative field slightly decreased in size; *ventral border of future yolk plug indicated by pigment concentration*.

Int. Crit.: Epibolic extension of presumptive ectoderm, particularly of its inner, sensorial layer, and reduction of vegetative area. Marked progress in formation of definitive mesodermal mantle. *which has extended up to equator at dorsal side and is forming all around vegetative area*. *First contact between prechordal part of archenteron roof and most caudal portion of sensorial layer of ectoderm*. *Invagination of short slit-shaped archenteron over 10-15°*. Clear topological and temporal distinction between rolling-in of inner marginal zone and invagination of archenteron.