PROFESSOR P.M. MOTTA ET AL / SCIENCE PHOTO LIBRARY PROFESSOR P.M. MOTTA ET AL / SCIENCE PHOTO LIBRARY
Two-cell embryo. Coloured light micrograph of a human embryo at the 2-cell stage. It is seen 40 hours after fertilisation. The cells are coloured green, surrounded by the zona pellucida layer (orange). In the zona pellucida can be seen the heads (small black dots) of sperm. At this stage each cell of the embryo is called a blastomere. Inclusions (dark green) inside each cell are the cellular debris after cell division. The two-cell stage is a primitive embryo, following the first mitotic division. These cells will continue to divide to form a human foetus composed of millions of cells. Magnification: x200 at 6x7cm size.
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