STAGE 1. Age 0 hours; length 1.4 - 1.5 mm.

Ext. Crit.: *One cell stage, shortly after fertilization*. Pigmentation darker verntrally than dorsally.

Int. Crit.: *Germinal vesicle disappeared*. Well defined cortical layer and layer of subcortical plasm, mainly located in animal half of egg. Inner plasm composed of inner animal, central and inner vegetative plasms. Special cytoplasmic inclusions near vegetative pole. Thickness of cortical and subcortical layers decreasing in animal-vegetative and dorso-ventral directions: *clear animal-vegetative and dorso-ventral polarity*.