STAGE 3. Age 2 hours; length 1.4 - 1.5 mm.

Ext. Crit.: *Advanced four cell stage*. *Second cleavage groove has reached vegetative pole*. In animal view dorsal blastomeres usually smaller than ventral ones; the latter darker than the former.

Int. Crit.: Partition wall between the four blastomeres. *Cleavage cavity present for the first time*, mainly in animal half of egg.