DAVID SCHARF / SCIENCE PHOTO LIBRARY. DAVID SCHARF / SCIENCE PHOTO LIBRARY.
Light micrograph of a mouse embryo, taken at the end of the 2nd mitotic division of the single cell after fertilisation. From now on the 2 cells will divide into 4, then 4 into 8, and so on. As the number of cells, called blastomeres, increases, the individual blastomeres become smaller, so that the overall size of the cellular aggregate does not increase at first. The well-defined exterior wall around the embryo is the zona pellucida, a protective glycoprotein layer.
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