EDELMANN / SCIENCE PHOTO LIBRARY EDELMANN / SCIENCE PHOTO LIBRARY
Light micrograph of a human zygote (the primitive embryo) at the single cell stage, that is, the ovum (egg) immediately after fertilisation by a single sperm. The fertilised ovum is preparing for its first mitotic division, the first stage in the formation of a hollow ball of cells (the blastocyst) with a localised thickening (called the inner cell mass) that becomes embedded into the uterine wall. The small mass of material immediately above the cell is a parcel of redundant chromosomes, ejected from the oocyte (the female sex cell) prior to fertilisation.
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