EDELMANN / SCIENCE PHOTO LIBRARY EDELMANN / SCIENCE PHOTO LIBRARY
Light micrograph of a human zygote (the primitive embryo) at the three-cell stage, that is, after two cleavage divisions - an early stage in the transformation from a single cell organism to one composed of millions of cells. The cells formed in these repeated cleavage operations are called blastomeres; 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 in the womb determines the subsequent position of the placenta.
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