STAGE 10 ¼. Age 10 h.; length 1.4 - 1.5 mm.

Ext. Crit.: *Early gastrula stage*. *First formation of dorsal blastopore groove*; groove still straight.

Int. Crit.: *Initial invagination of archenteron*. *Extension of inner blastopore lip from dorsal to lateral, and even partially to ventral side of inner marginal zone*. Gradual delimitation of prechordal portion of definitive mesodermal mantle from central entoderm mass.