Molecular and Developmental Biology Laboratory Fertilization, Grey Crescent, Blastulation, Gastrulation in Frog Establishment of polarity.
Polarization exists in the ovum with the animal and vegetal halves.
A useful generalization for amphibia is that the region of the animal pole will ultimately form the head and the vegetal pole, the tail. Fertilization further establishes the polar axes within the egg. The grey crescent, a localized crescentic area, appears along the border between the darkly-pigmented animal pole cytoplasm and the lighter vegetal pole cytoplasm of the fertilized egg. So even before cleavage has begun the three principal axes of the amphibian embryo are established. Include the grey crescent, cortical granules, fertilization membranes, polarity within the maturing oocyte and the pigmented cortex shift after sperm penetration.