During embryonic development, transformation of the flat, two-layer germinal disc into a three-dimensional body depends on transition of some cells from an mesenchymal phenotype.
2.
Polyspermy is apparently allowed; when the yolk passes through the infundibulum of the oviduct and is exposed to sperm storage glands, the sperm preferentially seek out the near center ( but not the exact center ) of the germinal disc region and break through the perivitelline membrane.
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