BSIP VEM / SCIENCE PHOTO LIBRARY BSIP VEM / SCIENCE PHOTO LIBRARY
Eight-cell embryo. Artwork illustrating a primitive human embryo composed of 8 cells, approximately two days after fertilisation. The 8 cells are surrounded by the zona pellucida, a thick spongy layer originally surrounding the ovum. The 8-cell stage follows the initial mitotic divisions which ultimately transform a single- celled organism into one composed of millions of cells. Subsequent division of these 8 cells (called blastomeres) results in the formation of a hollow ball of cells (the blastocyst) part of which will eventually develop into the foetus. Formation of the blastocyst occurs before the embryo implants into the uterine wall.
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