RICHARD G. RAWLINS / CUSTOM MEDICAL STOCK PHOTO / SCIENCE PHOTO LIBRARY RICHARD G. RAWLINS / CUSTOM MEDICAL STOCK PHOTO / SCIENCE PHOTO LIBRARY
Two-cell embryo. Light micrograph of a human embryo at the 2-cell stage of development (about 24 hours after fertilisation). Termed a primitive embryo, and composed of just two cells following its first mitotic division, this is the initial transformation from a single cell to an organism composed of millions of cells. The process of division is known as cleavage. These two cells (called blastomeres) will continue to cleave, but the cells do not grow between divisions and so decrease in size. This mitotic process speeds up as more cells divide; it will take some 8 weeks before a truly human shape is formed.
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