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A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation

그런데 grey crescent은 morphogen이라는 물질 이 아니라, 그 물질이 non uniformly 모이는 region이라고 생각하면 되어요. 

A cortical region of the newly fertilised egg of frogs and some salamanders that forms just after fertilization on the side opposite sperm penetration.

 Grey Crescent is a morphogenetic field that determined the fate of subsequent cells.