STAGE 10. Age 9 h.; length 1.4 - 1.5 mm.

Ext. Crit.:*Initial gastrula stage*. *First indication of blastopore only by pigment concentration*. No formation of groove.

Int. Crit.: *Formation of bottle-necked cells in outer cell layer in area of future blastopore groove*. *Beginning of formation of mesodermal mantle at dorsal side* by rolling-in of presumptive prechordal plate mesoderm around the inner blastopore lip.