PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Fertilised "polyspermic" egg. Light micrograph of a human egg cell after multiple fertilisation (polyspermy or polyspermia). Two spermatozoa (sperm) have penetrated the cell, rather than one sperm as is usually the case, producing this abnormal zygote. Inside the cell are three rounded pronuclei: two sperm pronuclei and one egg pronucleus. The cell will develop into an embryo with too many chromosomes, causing it to be spontaneously aborted. The egg cell's outer layer, the zona pellucida, encloses a polar body (lower centre). Magnification unknown.
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