MICHEL DELARUE, ISM / SCIENCE PHOTO LIBRARY MICHEL DELARUE, ISM / SCIENCE PHOTO LIBRARY
Xenopus laevis embryo, four-cell stage. Xenopus laevis, the African clawed frog, is widely used as a model organism in research in developmental biology. It is popular in this field as its egg cells are large and easily manipulated in the laboratory. The four cells (blastomeres) here have resulted from two phases of cell division (mitosis): the fertilised egg (zygote) divided first into two identical cells, then each divided again. The cells do not grow between divisions, so the four-cell embryo is around the same size as the initial egg cell. Magnification: x55 when printed 10 centimetres wide.
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