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Cleavage is the rapid mitotic cell divisions that immediately follow fertilization. As the number of cells increases to about 16 - 64, the embryo is called a morula. Cleavage does not increase the size of the embryo. Thus, the cells become smaller as their number increases. All cells in a morula look similar. However, they may contain different cytoplasmic materials due to cytoplasm rearrangement at the egg activation. These subtle differences are important for subsequent cell differentiation.
The early embryo: (a) Two-cell stage. (b) Four-cell stage. (c) Eight-cell stage. (d, e) Morula stage. z.p. zona pellucida. p.gl. Polar bodies. [From Wikipedia]
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