Cortical rotation and the grey crescent. Sperm (purple) entry triggers the rotation of the cortex of the cell relative to the inner cell mass. This rotation moves determinants on the vegetal (yolky) side of the egg (green) towards the animal/vegetal border. A grey "cresent" appears where the clear vegetal cortex overlies the grey animal inner cell mass. The animal cortex is darkly pigmented, hiding the inner cell mass underneath it. The grey crescent is the site of the future organizer that runs along the anterioposterior axis. If cortical rotation is disrupted, then this organizer is never formed and the embryo becomes a mass of ventral tissue (belly piece). If a second organizer is added to the embryo, a second axis is formed, leading to twinning. An extreme case of this is a complete twinned tadpole.
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xenopus, grey crescent, fertilization, development, blastula, zygote, axes, evodevo, evolutionary developmental biology, microtubules