About 10 minutes after penetration of sperm, the yellow cortical ooplasm of egg starts flowing towards the sperm entrance. At the future dorsal side, the cytoplasm moves towards the animal pole, and at the opposite side (the future ventral side) it moves towards the vegetal pole. This exposes lightly pigmented crescent-shaped area of marginal cytoplasm on the dorsal side, excetly opposite, the point of sperm entrance. This is known as grey crescent.