BSIP ESTIOT / SCIENCE PHOTO LIBRARY BSIP ESTIOT / SCIENCE PHOTO LIBRARY
Two-cell embryo. Artwork based on a light micrograph of a human embryo composed of two cells, approximately 30 hours after fertilisation. The 2 cells are surrounded by the zona pellucida (yellow, bubbly), a thick spongy protective layer originally surrounding the ovum. The 2-cell stage follows the initial mitotic division which ultimately transforms a single-celled organism into one composed of millions of cells. Subsequent divisions of these 2 cells (called blastomeres) result in the formation of a hollow ball of cells (the blastocyst), which attaches to the uterine lining within a week. Part of the blastocyst will eventually develop into a foetus.
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