ANDY WALKER, MIDLAND FERTILITY SERVICES / SCIENCE PHOTO LIBRARY ANDY WALKER, MIDLAND FERTILITY SERVICES / SCIENCE PHOTO LIBRARY
Light micrograph of a human zygote (the primitive embryo), composed of two cells following its first mitotic division - the initial transformation from a single cell organism to one composed of millions of cells. The two cells (called blastomeres) both undergo similar cleavage; successive divisions result in the formation of a hollow ball of cells (the blastocyst) with a localised thickening (the inner cell mass) that will develop into the actual embryo. Development of the blastocyst occurs before it implants into the uterine wall. The site of implantation determines the subsequent position of the placenta.
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