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 pink, surrounded by the zona pellucida layer (purple). In the zona pellucida can be seen the heads (small black dots) of sperm. At lower frame is a mass of corona cumulus cells which once completely surrounded the unfertilised egg. At the two-cell stage, this is a primitive embryo, following the first mitotic division. Each cell is called a blastomere and will continue to divide and transform into a human composed of millions of cells. Magnification: x170 at 6x7cm size.
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